Creating the Singleton design pattern in PHP5

Question

How would one create a Singleton class using PHP5 classes

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protected  static   instance   public static function getInstance         if is null self    instance                 self    instance   new self              return self    instance      This code can apply for any class without caring about its class name

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I liked  jose-segura method of using traits but didn t like the need to define a static variable on sub-classes  Below is a solution that avoids it by caching the instances in a static local variable to the factory method indexed by class name    lt  php trait Singleton        Single point of entry for creating a new instance  For a given     class always returns the same instance    public static function instance        static  instances   array         class   get called class        if   isset  instances  class      instances  class    new  class        return  instances  class            Kill traditional methods of creating new instances   protected function   clone        protected function   construct          Usage is the same as  jose-segura only no need for the static variable in sub-classes

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PHP 5 3 allows the creation of an inheritable Singleton class via late static binding   class Singleton       protected static  instance   null       protected function   construct                   Thou shalt not construct that which is unconstructable             protected function   clone                   Me not like clones  Me smash clones             public static function getInstance                 if   isset static   instance                 static   instance   new static                    return static   instance            This solves the problem  that prior to PHP 5 3 any class that extended a Singleton would produce an instance of its parent class instead of its own   Now you can do   class Foobar extends Singleton      foo   Foobar  getInstance      And  foo will be an instance of Foobar instead of an instance of Singleton

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Singleton class        final class UserFactory                  Call this method to get singleton                return UserFactory             public static function Instance                 static  inst   null          if   inst     null                 inst   new UserFactory                      return  inst                        Private ctor so nobody else can instantiate it                    private function   construct                    To use    fact   UserFactory  Instance     fact2   UserFactory  Instance       fact     fact2   But    fact   new UserFactory     Throws an error   See http   php net manual en language variables scope php language variables scope static to understand static variable scopes and why setting static  inst   null  works

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Database class that checks if there is any existing database instance it will return previous instance      class Database             public static  instance             public static function getInstance                  if  isset Database   instance                        Database   instance   new Database                                 return Database   instance                          private function   cunstruct                     private and cant create multiple objects                            public function getQuery                  return  Test Query Data                               dbObj   Database  getInstance           dbObj2   Database  getInstance          var dump  dbObj         var dump  dbObj2           After execution you will get following output    object Database  1   object Database  1           Ref http   www phptechi com php-singleton-design-patterns-example html

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class Database             variable to hold db connection         private  db            note we used static variable beacuse an instance cannot be used to refer this         public static  instance             note constructor is private so that classcannot be instantiated         private function   construct                code connect to database                                to prevent loop hole in PHP so that the class cannot be cloned         private function   clone                 used static function so that  this can be called from other classes         public static function getInstance                 if    self   instance instanceof self                     self   instance   new self                                          return self   instance                      public function query  sql                 code to run the query                    Access the method getInstance using  db   Singleton  getInstance     db- gt query

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Singleton class        final class UserFactory                  Call this method to get singleton                return UserFactory             public static function Instance                 static  inst   null          if   inst     null                 inst   new UserFactory                      return  inst                        Private ctor so nobody else can instantiate it                    private function   construct                    To use    fact   UserFactory  Instance     fact2   UserFactory  Instance       fact     fact2   But    fact   new UserFactory     Throws an error   See http   php net manual en language variables scope php language variables scope static to understand static variable scopes and why setting static  inst   null  works

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Here s my example that provides ability to call as  var   new Singleton   and also creating 3 variables to test if it creates new object    class Singleton       private static  data       function   construct            if   this   data    null                this- gt makeSingleton                      echo   lt br  gt    this   data             private function makeSingleton             this   data   rand 0  100              public function change  new val            this   data    new val             public function printme            echo   lt br  gt    this   data              a   new Singleton     b   new Singleton     c   new Singleton      a- gt change -2    a- gt printme     b- gt printme      d   new Singleton     d- gt printme

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Quick example   final class Singleton       private static  instance   null       private function   construct          private function   clone          private function   wakeup          public static function get instance                 if   static   instance     null                 static   instance   new static                      return static   instance            Hope help

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All this complexity   late static binding      harumph  is  to me  simply a sign of PHP s broken object class model  If class objects were first-class objects  see Python   then    instance  would be a class instance variable -- a member of the class object  as opposed to a member property of its instances  and also as opposed to shared by its descendants  In the Smalltalk world  this is the difference between a  class variable  and a  class instance variable    In PHP  it looks to me as though we need to take to heart the guidance that patterns are a guide towards writing code -- we might perhaps think about a Singleton template  but trying to write code that inherits from an actual  Singleton  class looks misguided for PHP  though I supposed some enterprising soul could create a suitable SVN keyword    I will continue to just code each singleton separately  using a shared template   Notice that I m absolutely staying OUT of the singletons-are-evil discussion  life is too short

User · Answer

Quick example   final class Singleton       private static  instance   null       private function   construct          private function   clone          private function   wakeup          public static function get instance                 if   static   instance     null                 static   instance   new static                      return static   instance            Hope help

User · Answer

I know this is probably going to cause an unnecessary flame war  but I can see how you might want more than one database connection  so I would concede the point that singleton might not be the best solution for that    however  there are other uses of the singleton pattern that I find extremely useful   Here s an example   I decided to roll my own MVC and templating engine because I wanted something really lightweight   However  the data that I want to display contains a lot of special math characters such as  ge  and  mu  and what have you    The data is stored as the actual UTF-8 character in my database rather than pre-HTML-encoded because my app can deliver other formats such as PDF and CSV in addition to HTML  The appropriate place to format for HTML is inside the template   view  if you will  that is responsible for rendering that page section  snippet   I want to convert them to their appropriate HTML entities  but PHPs get html translation table   function is not super fast   It makes better sense to retrieve the data one time and store as an array  making it available for all to use   Here s a sample I knocked together to test the speed   Presumably  this would work regardless of whether the other methods you use  after getting the instance  were static or not   class EncodeHTMLEntities        private static  instance   null   stores the instance of self     private  r   null   array of chars elligalbe for replacement      private function   clone           disable cloning  no reason to clone      private function   construct                  allEntities   get html translation table HTML ENTITIES  ENT NOQUOTES            specialEntities   get html translation table HTML SPECIALCHARS  ENT NOQUOTES            this- gt r   array diff  allEntities   specialEntities              public static function replace  string                if   self   instance instanceof self                 self   instance   new self                      return strtr  string  self   instance- gt r             test one million encodings of a string  start   microtime true   for  x 0   x lt 1000000   x          dump   EncodeHTMLEntities  replace  Reference method for diagnosis of CDAD  but clinical usefulness limited due to extended turnaround time   96 hrs        end   microtime true   echo  Run time      end- start    seconds using singleton n     now repeat the same without using singleton  start   microtime true   for  x 0   x lt 1000000   x          allEntities   get html translation table HTML ENTITIES  ENT NOQUOTES        specialEntities   get html translation table HTML SPECIALCHARS  ENT NOQUOTES        r   array diff  allEntities   specialEntities        dump   strtr  Reference method for diagnosis of CDAD  but clinical usefulness limited due to extended turnaround time   96 hrs     r      end   microtime true   echo  Run time      end- start    seconds without using singleton     Basically  I saw typical results like this   php test php Run time  27 842966794968 seconds using singleton Run time  237 78191494942 seconds without using singleton   So while I m certainly no expert  I don t see a more convenient and reliable way to reduce the overhead of slow calls for some kind of data  while making it super simple  single line of code to do what you need    Granted my example only has one useful method  and therefore is no better than a globally defined function  but as soon as you have two methods  you re going to want to group them together  right   Am I way off base   Also  I prefer examples that actually DO something  since sometimes it s hard to visualise when an example includes statements like    do something useful here  which I see all the time when searching for tutorials   Anyway  I d love any feedback or comments on why using a singleton for this type of thing is detrimental  or overly complicated

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This should be the right way of Singleton   class Singleton        private static  instance      private  count   0       protected function   construct                public static function singleton             if   isset self   instance                  self   instance   new Singleton                      return self   instance              public function increment                 return  this- gt count               protected function   clone                protected function   wakeup

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Singleton class        final class UserFactory                  Call this method to get singleton                return UserFactory             public static function Instance                 static  inst   null          if   inst     null                 inst   new UserFactory                      return  inst                        Private ctor so nobody else can instantiate it                    private function   construct                    To use    fact   UserFactory  Instance     fact2   UserFactory  Instance       fact     fact2   But    fact   new UserFactory     Throws an error   See http   php net manual en language variables scope php language variables scope static to understand static variable scopes and why setting static  inst   null  works

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lt  php        Singleton patter in php      trait SingletonTrait      protected static  inst   null              call this method to get instance           public static function getInstance          if  static   inst     null            static   inst   new static                  return static   inst                  protected to prevent clonning           protected function   clone                    protected so no one else can instance it           protected function   construct            to use           example of class definitions using SingletonTrait     class DBFactory              we are adding the trait here            use SingletonTrait              This class will have a single db connection as an example          protected  db             as an example we will create a PDO connection         protected function   construct         this- gt db            new PDO  mysql dbname foodb port 3305 host 127 0 0 1   foouser   foopass          class DBFactoryChild extends DBFactory              we repeating the inst so that it will differentiate it      from UserFactory singleton           protected static  inst   null             example of instanciating the classes      uf0   DBFactoryChild  getInstance    var dump  uf0    uf1   DBFactory  getInstance    var dump  uf1   echo  uf0      uf1    respose   object DBFactoryChild  1  0      object DBFactory  2  0        If you are using PHP 5 4  trait its an option  so you don t have to waste the inheritance hierarchy in order to have the Singleton pattern  and also notice that whether you use traits or extends Singleton class  one loose end was to create singleton of child classes if you dont add the following line of code      protected static  inst   null    in the child class  the unexpected result will be   object DBFactoryChild  1  0      object DBFactoryChild  1  0

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Here s my example that provides ability to call as  var   new Singleton   and also creating 3 variables to test if it creates new object    class Singleton       private static  data       function   construct            if   this   data    null                this- gt makeSingleton                      echo   lt br  gt    this   data             private function makeSingleton             this   data   rand 0  100              public function change  new val            this   data    new val             public function printme            echo   lt br  gt    this   data              a   new Singleton     b   new Singleton     c   new Singleton      a- gt change -2    a- gt printme     b- gt printme      d   new Singleton     d- gt printme

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You probably should add a private   clone   method to disallow cloning of an instance   private function   clone        If you don t include this method the following gets possible   inst1 UserFactory  Instance       to stick with the example provided above  inst2 clone  inst1    now  inst1      inst2 - they are not the same instance any more

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You don t really need to use Singleton pattern because it s considered to be an antipattern  Basically there is a lot of reasons to not to implement this pattern at all  Read this to start with  Best practice on PHP singleton classes    If after all you still think you need to use Singleton pattern then we could write a class that will allow us to get Singleton functionality by extending our SingletonClassVendor abstract class   This is what I came with to solve this problem    lt  php namespace wl            author DevWL     dosc allows only one instance for each extending class     it acts a litle bit as registry from the SingletonClassVendor abstract class point of view    but it provides a valid singleton behaviour for its children classes    Be aware  the singleton pattern is consider to be an anti-pattern    mostly because it can be hard to debug and it comes with some limitations     In most cases you do not need to use singleton pattern    so take a longer moment to think about it before you use it      abstract class SingletonClassVendor                   holds an single instance of the child class                 var array of objects             protected static  instance                         desc provides a single slot to hold an instance interchanble between all child classes           return object             public static final function getInstance             class   get called class       or get class new static             if  isset self   instance  class       self   instance  class  instanceof  class               self   instance  class    new static       create and instance of child class which extends Singleton super class             echo  new     class   PHP EOL     remove this line after testing             return  self   instance  class      remove this line after testing                   echo  old     class   PHP EOL     remove this line after testing         return static   instance  class                         Make constructor abstract to force protected implementation of the   constructor   method  so that nobody can call directly  new Class                 abstract protected function   construct                    Make clone magic method private  so nobody can clone instance              private function   clone                      Make sleep magic method private  so nobody can serialize instance              private function   sleep                      Make wakeup magic method private  so nobody can unserialize instance              private function   wakeup           Use example          EXAMPLE               example 1 - Database class by extending SingletonClassVendor abstract class becomes fully functional singleton       constructor must be set to protected becaouse        1 to allow instansiation from parent class       2 to prevent direct instanciation of object with  new  keword       3 to meet requierments of SingletonClassVendor abstract class     class Database extends SingletonClassVendor       public  type    SomeClass       protected function   construct            echo  DDDDDDDDD   PHP EOL     remove this line after testing                    example 2 - Config         class Config extends SingletonClassVendor       public  name    Config       protected function   construct            echo  CCCCCCCCCC    PHP EOL     remove this line after testing           Just to prove that it works as expected            TESTING      bd1   Database  getInstance       new  bd2   Database  getInstance       old  bd3   Config  getInstance       new  bd4   Config  getInstance       old  bd5   Config  getInstance       old  bd6   Database  getInstance       old  bd7   Database  getInstance       old  bd8   Config  getInstance       old  echo PHP EOL  COMPARE ALL DATABASE INSTANCES  PHP EOL  var dump  bd1   echo   bd1      bd2      bd1      bd2     TRUE    PHP EOL    FALSE    PHP EOL     TRUE echo   bd2      bd6      bd2      bd6     TRUE    PHP EOL    FALSE    PHP EOL     TRUE echo   bd6      bd7      bd6      bd7     TRUE    PHP EOL    FALSE    PHP EOL     TRUE  echo PHP EOL   echo PHP EOL  COMPARE ALL CONFIG INSTANCES   PHP EOL  var dump  bd3   echo   bd3      bd4      bd3      bd4     TRUE    PHP EOL    FALSE    PHP EOL     TRUE echo   bd4      bd5      bd4      bd5     TRUE    PHP EOL    FALSE    PHP EOL     TRUE echo   bd5      bd8      bd5      bd8     TRUE    PHP EOL    FALSE    PHP EOL     TRUE

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class Database             variable to hold db connection         private  db            note we used static variable beacuse an instance cannot be used to refer this         public static  instance             note constructor is private so that classcannot be instantiated         private function   construct                code connect to database                                to prevent loop hole in PHP so that the class cannot be cloned         private function   clone                 used static function so that  this can be called from other classes         public static function getInstance                 if    self   instance instanceof self                     self   instance   new self                                          return self   instance                      public function query  sql                 code to run the query                    Access the method getInstance using  db   Singleton  getInstance     db- gt query

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Unfortunately Inwdr s answer breaks when there are multiple subclasses   Here is a correct inheritable Singleton base class   class Singleton       private static  instances   array        protected function   construct          protected function   clone          public function   wakeup                 throw new Exception  Cannot unserialize singleton               public static function getInstance                  cls   get called class       late-static-bound class name         if   isset self   instances  cls                  self   instances  cls    new static                    return self   instances  cls             Test code   class Foo extends Singleton    class Bar extends Singleton     echo get class Foo  getInstance        n   echo get class Bar  getInstance        n

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This should be the right way of Singleton   class Singleton        private static  instance      private  count   0       protected function   construct                public static function singleton             if   isset self   instance                  self   instance   new Singleton                      return self   instance              public function increment                 return  this- gt count               protected function   clone                protected function   wakeup

User · Answer

I know this is probably going to cause an unnecessary flame war  but I can see how you might want more than one database connection  so I would concede the point that singleton might not be the best solution for that    however  there are other uses of the singleton pattern that I find extremely useful   Here s an example   I decided to roll my own MVC and templating engine because I wanted something really lightweight   However  the data that I want to display contains a lot of special math characters such as  ge  and  mu  and what have you    The data is stored as the actual UTF-8 character in my database rather than pre-HTML-encoded because my app can deliver other formats such as PDF and CSV in addition to HTML  The appropriate place to format for HTML is inside the template   view  if you will  that is responsible for rendering that page section  snippet   I want to convert them to their appropriate HTML entities  but PHPs get html translation table   function is not super fast   It makes better sense to retrieve the data one time and store as an array  making it available for all to use   Here s a sample I knocked together to test the speed   Presumably  this would work regardless of whether the other methods you use  after getting the instance  were static or not   class EncodeHTMLEntities        private static  instance   null   stores the instance of self     private  r   null   array of chars elligalbe for replacement      private function   clone           disable cloning  no reason to clone      private function   construct                  allEntities   get html translation table HTML ENTITIES  ENT NOQUOTES            specialEntities   get html translation table HTML SPECIALCHARS  ENT NOQUOTES            this- gt r   array diff  allEntities   specialEntities              public static function replace  string                if   self   instance instanceof self                 self   instance   new self                      return strtr  string  self   instance- gt r             test one million encodings of a string  start   microtime true   for  x 0   x lt 1000000   x          dump   EncodeHTMLEntities  replace  Reference method for diagnosis of CDAD  but clinical usefulness limited due to extended turnaround time   96 hrs        end   microtime true   echo  Run time      end- start    seconds using singleton n     now repeat the same without using singleton  start   microtime true   for  x 0   x lt 1000000   x          allEntities   get html translation table HTML ENTITIES  ENT NOQUOTES        specialEntities   get html translation table HTML SPECIALCHARS  ENT NOQUOTES        r   array diff  allEntities   specialEntities        dump   strtr  Reference method for diagnosis of CDAD  but clinical usefulness limited due to extended turnaround time   96 hrs     r      end   microtime true   echo  Run time      end- start    seconds without using singleton     Basically  I saw typical results like this   php test php Run time  27 842966794968 seconds using singleton Run time  237 78191494942 seconds without using singleton   So while I m certainly no expert  I don t see a more convenient and reliable way to reduce the overhead of slow calls for some kind of data  while making it super simple  single line of code to do what you need    Granted my example only has one useful method  and therefore is no better than a globally defined function  but as soon as you have two methods  you re going to want to group them together  right   Am I way off base   Also  I prefer examples that actually DO something  since sometimes it s hard to visualise when an example includes statements like    do something useful here  which I see all the time when searching for tutorials   Anyway  I d love any feedback or comments on why using a singleton for this type of thing is detrimental  or overly complicated

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The Real One and Modern way to make Singleton Pattern is    lt  php         Singleton Pattern         Modern implementation      class Singleton                  Call this method to get singleton             public static function instance               static  instance   false        if   instance     false                      Late static binding  PHP 5 3            instance   new static                   return  instance                        Make constructor private  so nobody can call  new Class               private function   construct                      Make clone magic method private  so nobody can clone instance              private function   clone                      Make sleep magic method private  so nobody can serialize instance              private function   sleep                      Make wakeup magic method private  so nobody can unserialize instance              private function   wakeup           So now you can use it like    lt  php         Database        Inherited from Singleton  so it s now got singleton behavior      class Database extends Singleton      protected  label              Example of that singleton is working correctly          public function setLabel  label           this- gt label    label         public function getLabel           return  this- gt label             create first instance  database   Database  instance     database- gt setLabel  Abraham    echo  database- gt getLabel     PHP EOL      now try to create other instance as well  other db   Database  instance    echo  other db- gt getLabel     PHP EOL     Abraham   other db- gt setLabel  Priler    echo  database- gt getLabel     PHP EOL     Priler echo  other db- gt getLabel     PHP EOL     Priler   As you see this realization is lot more flexible

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This is the example of create singleton on Database class    design patterns 1  singleton  class Database    public static  instance    public static function getInstance        if  isset Database   instance        Database   instance new Database          return Database   instance                 db Database  getInstance       db2 Database  getInstance       db3 Database  getInstance       var dump  db     var dump  db2     var dump  db3     then out put is --    object Database  1    object Database  1    object Database  1    use only single instance not create 3 instance

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You don t really need to use Singleton pattern because it s considered to be an antipattern  Basically there is a lot of reasons to not to implement this pattern at all  Read this to start with  Best practice on PHP singleton classes    If after all you still think you need to use Singleton pattern then we could write a class that will allow us to get Singleton functionality by extending our SingletonClassVendor abstract class   This is what I came with to solve this problem    lt  php namespace wl            author DevWL     dosc allows only one instance for each extending class     it acts a litle bit as registry from the SingletonClassVendor abstract class point of view    but it provides a valid singleton behaviour for its children classes    Be aware  the singleton pattern is consider to be an anti-pattern    mostly because it can be hard to debug and it comes with some limitations     In most cases you do not need to use singleton pattern    so take a longer moment to think about it before you use it      abstract class SingletonClassVendor                   holds an single instance of the child class                 var array of objects             protected static  instance                         desc provides a single slot to hold an instance interchanble between all child classes           return object             public static final function getInstance             class   get called class       or get class new static             if  isset self   instance  class       self   instance  class  instanceof  class               self   instance  class    new static       create and instance of child class which extends Singleton super class             echo  new     class   PHP EOL     remove this line after testing             return  self   instance  class      remove this line after testing                   echo  old     class   PHP EOL     remove this line after testing         return static   instance  class                         Make constructor abstract to force protected implementation of the   constructor   method  so that nobody can call directly  new Class                 abstract protected function   construct                    Make clone magic method private  so nobody can clone instance              private function   clone                      Make sleep magic method private  so nobody can serialize instance              private function   sleep                      Make wakeup magic method private  so nobody can unserialize instance              private function   wakeup           Use example          EXAMPLE               example 1 - Database class by extending SingletonClassVendor abstract class becomes fully functional singleton       constructor must be set to protected becaouse        1 to allow instansiation from parent class       2 to prevent direct instanciation of object with  new  keword       3 to meet requierments of SingletonClassVendor abstract class     class Database extends SingletonClassVendor       public  type    SomeClass       protected function   construct            echo  DDDDDDDDD   PHP EOL     remove this line after testing                    example 2 - Config         class Config extends SingletonClassVendor       public  name    Config       protected function   construct            echo  CCCCCCCCCC    PHP EOL     remove this line after testing           Just to prove that it works as expected            TESTING      bd1   Database  getInstance       new  bd2   Database  getInstance       old  bd3   Config  getInstance       new  bd4   Config  getInstance       old  bd5   Config  getInstance       old  bd6   Database  getInstance       old  bd7   Database  getInstance       old  bd8   Config  getInstance       old  echo PHP EOL  COMPARE ALL DATABASE INSTANCES  PHP EOL  var dump  bd1   echo   bd1      bd2      bd1      bd2     TRUE    PHP EOL    FALSE    PHP EOL     TRUE echo   bd2      bd6      bd2      bd6     TRUE    PHP EOL    FALSE    PHP EOL     TRUE echo   bd6      bd7      bd6      bd7     TRUE    PHP EOL    FALSE    PHP EOL     TRUE  echo PHP EOL   echo PHP EOL  COMPARE ALL CONFIG INSTANCES   PHP EOL  var dump  bd3   echo   bd3      bd4      bd3      bd4     TRUE    PHP EOL    FALSE    PHP EOL     TRUE echo   bd4      bd5      bd4      bd5     TRUE    PHP EOL    FALSE    PHP EOL     TRUE echo   bd5      bd8      bd5      bd8     TRUE    PHP EOL    FALSE    PHP EOL     TRUE

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You probably should add a private   clone   method to disallow cloning of an instance   private function   clone        If you don t include this method the following gets possible   inst1 UserFactory  Instance       to stick with the example provided above  inst2 clone  inst1    now  inst1      inst2 - they are not the same instance any more

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Unfortunately Inwdr s answer breaks when there are multiple subclasses   Here is a correct inheritable Singleton base class   class Singleton       private static  instances   array        protected function   construct          protected function   clone          public function   wakeup                 throw new Exception  Cannot unserialize singleton               public static function getInstance                  cls   get called class       late-static-bound class name         if   isset self   instances  cls                  self   instances  cls    new static                    return self   instances  cls             Test code   class Foo extends Singleton    class Bar extends Singleton     echo get class Foo  getInstance        n   echo get class Bar  getInstance        n

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Database class that checks if there is any existing database instance it will return previous instance      class Database             public static  instance             public static function getInstance                  if  isset Database   instance                        Database   instance   new Database                                 return Database   instance                          private function   cunstruct                     private and cant create multiple objects                            public function getQuery                  return  Test Query Data                               dbObj   Database  getInstance           dbObj2   Database  getInstance          var dump  dbObj         var dump  dbObj2           After execution you will get following output    object Database  1   object Database  1           Ref http   www phptechi com php-singleton-design-patterns-example html

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I have written long back thought to share here   class SingletonDesignPattern          just for demo there will be only one instance     private static  instanceCount  0         create the private instance variable     private static  myInstance null         make constructor private so no one create object using new Keyword     private function    construct            no one clone the object     private function    clone            avoid serialazation     public function   wakeup            ony one way to create  object     public static  function  getInstance             if self   myInstance  null               self   myInstance new SingletonDesignPattern                self   instanceCount                      return self   myInstance             public static function getInstanceCount            return self   instanceCount              now lets play with singleton design pattern   instance   SingletonDesignPattern  getInstance     instance   SingletonDesignPattern  getInstance     instance   SingletonDesignPattern  getInstance     instance   SingletonDesignPattern  getInstance     echo  number of instances    SingletonDesignPattern  getInstanceCount

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All this complexity   late static binding      harumph  is  to me  simply a sign of PHP s broken object class model  If class objects were first-class objects  see Python   then    instance  would be a class instance variable -- a member of the class object  as opposed to a member property of its instances  and also as opposed to shared by its descendants  In the Smalltalk world  this is the difference between a  class variable  and a  class instance variable    In PHP  it looks to me as though we need to take to heart the guidance that patterns are a guide towards writing code -- we might perhaps think about a Singleton template  but trying to write code that inherits from an actual  Singleton  class looks misguided for PHP  though I supposed some enterprising soul could create a suitable SVN keyword    I will continue to just code each singleton separately  using a shared template   Notice that I m absolutely staying OUT of the singletons-are-evil discussion  life is too short

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This article covers topic quite extensively   http   www phptherightway com pages Design-Patterns html singleton     Note the following          The constructor   construct   is declared as protected to prevent    creating a new instance outside of the class via the new operator    The magic method   clone   is declared as private to prevent cloning    of an instance of the class via the clone operator    The magic method   wakeup   is declared as private to prevent unserializing of an instance of the class via the global function   unserialize      A new instance is created via late static binding in the static creation    method getInstance   with the keyword static  This   allows the subclassing of the class Singleton in the example

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Supports Multiple Objects with 1 line per class  This method will enforce singletons on any class you wish  al you have to do is add 1 method to the class you wish to make a singleton and this will do it for you  This also stores objects in a  quot SingleTonBase quot  class so you can debug all your objects that you have used in your system by recursing the SingleTonBase objects   Create a file called SingletonBase php and include it in root of your script  The code is abstract class SingletonBase       private static  storage   array         public static function Singleton  class                if in array  class self   storage                         return self   storage  class                     return self   storage  class    new  class              public static function storage                return self   storage           Then for any class you want to make a singleton just add this small single method  public static function Singleton         return SingletonBase  Singleton get class         Here is a small example  include  libraries SingletonBase resource php    class Database         Add that singleton function      public static function Singleton                 return SingletonBase  Singleton get class                public function run                 echo  running                Database   Database  Singleton      Database- gt run     And you can just add this singleton function in any class you have and it will only create 1 instance per class  NOTE  You should always make the   construct private to eliminate the use of new Class    instantiations

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PHP 5 3 allows the creation of an inheritable Singleton class via late static binding   class Singleton       protected static  instance   null       protected function   construct                   Thou shalt not construct that which is unconstructable             protected function   clone                   Me not like clones  Me smash clones             public static function getInstance                 if   isset static   instance                 static   instance   new static                    return static   instance            This solves the problem  that prior to PHP 5 3 any class that extended a Singleton would produce an instance of its parent class instead of its own   Now you can do   class Foobar extends Singleton      foo   Foobar  getInstance      And  foo will be an instance of Foobar instead of an instance of Singleton

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Singleton class        final class UserFactory                  Call this method to get singleton                return UserFactory             public static function Instance                 static  inst   null          if   inst     null                 inst   new UserFactory                      return  inst                        Private ctor so nobody else can instantiate it                    private function   construct                    To use    fact   UserFactory  Instance     fact2   UserFactory  Instance       fact     fact2   But    fact   new UserFactory     Throws an error   See http   php net manual en language variables scope php language variables scope static to understand static variable scopes and why setting static  inst   null  works

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protected  static   instance   public static function getInstance         if is null self    instance                 self    instance   new self              return self    instance      This code can apply for any class without caring about its class name

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I have written long back thought to share here   class SingletonDesignPattern          just for demo there will be only one instance     private static  instanceCount  0         create the private instance variable     private static  myInstance null         make constructor private so no one create object using new Keyword     private function    construct            no one clone the object     private function    clone            avoid serialazation     public function   wakeup            ony one way to create  object     public static  function  getInstance             if self   myInstance  null               self   myInstance new SingletonDesignPattern                self   instanceCount                      return self   myInstance             public static function getInstanceCount            return self   instanceCount              now lets play with singleton design pattern   instance   SingletonDesignPattern  getInstance     instance   SingletonDesignPattern  getInstance     instance   SingletonDesignPattern  getInstance     instance   SingletonDesignPattern  getInstance     echo  number of instances    SingletonDesignPattern  getInstanceCount

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I liked  jose-segura method of using traits but didn t like the need to define a static variable on sub-classes  Below is a solution that avoids it by caching the instances in a static local variable to the factory method indexed by class name    lt  php trait Singleton        Single point of entry for creating a new instance  For a given     class always returns the same instance    public static function instance        static  instances   array         class   get called class        if   isset  instances  class      instances  class    new  class        return  instances  class            Kill traditional methods of creating new instances   protected function   clone        protected function   construct          Usage is the same as  jose-segura only no need for the static variable in sub-classes

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This is the example of create singleton on Database class    design patterns 1  singleton  class Database    public static  instance    public static function getInstance        if  isset Database   instance        Database   instance new Database          return Database   instance                 db Database  getInstance       db2 Database  getInstance       db3 Database  getInstance       var dump  db     var dump  db2     var dump  db3     then out put is --    object Database  1    object Database  1    object Database  1    use only single instance not create 3 instance

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lt  php        Singleton patter in php      trait SingletonTrait      protected static  inst   null              call this method to get instance           public static function getInstance          if  static   inst     null            static   inst   new static                  return static   inst                  protected to prevent clonning           protected function   clone                    protected so no one else can instance it           protected function   construct            to use           example of class definitions using SingletonTrait     class DBFactory              we are adding the trait here            use SingletonTrait              This class will have a single db connection as an example          protected  db             as an example we will create a PDO connection         protected function   construct         this- gt db            new PDO  mysql dbname foodb port 3305 host 127 0 0 1   foouser   foopass          class DBFactoryChild extends DBFactory              we repeating the inst so that it will differentiate it      from UserFactory singleton           protected static  inst   null             example of instanciating the classes      uf0   DBFactoryChild  getInstance    var dump  uf0    uf1   DBFactory  getInstance    var dump  uf1   echo  uf0      uf1    respose   object DBFactoryChild  1  0      object DBFactory  2  0        If you are using PHP 5 4  trait its an option  so you don t have to waste the inheritance hierarchy in order to have the Singleton pattern  and also notice that whether you use traits or extends Singleton class  one loose end was to create singleton of child classes if you dont add the following line of code      protected static  inst   null    in the child class  the unexpected result will be   object DBFactoryChild  1  0      object DBFactoryChild  1  0

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Supports Multiple Objects with 1 line per class  This method will enforce singletons on any class you wish  al you have to do is add 1 method to the class you wish to make a singleton and this will do it for you  This also stores objects in a  quot SingleTonBase quot  class so you can debug all your objects that you have used in your system by recursing the SingleTonBase objects   Create a file called SingletonBase php and include it in root of your script  The code is abstract class SingletonBase       private static  storage   array         public static function Singleton  class                if in array  class self   storage                         return self   storage  class                     return self   storage  class    new  class              public static function storage                return self   storage           Then for any class you want to make a singleton just add this small single method  public static function Singleton         return SingletonBase  Singleton get class         Here is a small example  include  libraries SingletonBase resource php    class Database         Add that singleton function      public static function Singleton                 return SingletonBase  Singleton get class                public function run                 echo  running                Database   Database  Singleton      Database- gt run     And you can just add this singleton function in any class you have and it will only create 1 instance per class  NOTE  You should always make the   construct private to eliminate the use of new Class    instantiations

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I agree with the first answer but I would also declare the class as final so that it cannot be extended as extending a singleton violates the singleton pattern   Also the instance variable should be private so that it cannot be accessed directly   Also make the   clone method private so that you cannot clone the singleton object   Below is some example code          Singleton class        final class UserFactory       private static   instance   null                  Private constructor                    private function   construct                      Private clone method                     private function   clone                      Call this method to get singleton                return UserFactory             public static function getInstance                 if  self    instance     null                self    instance   new UserFactory                      return self    instance            Example Usage   user factory   UserFactory  getInstance      What this stops you from doing  which would violate the singleton pattern      YOU CANNOT DO THIS    user factory   UserFactory    instance   class SecondUserFactory extends UserFactory

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This article covers topic quite extensively   http   www phptherightway com pages Design-Patterns html singleton     Note the following          The constructor   construct   is declared as protected to prevent    creating a new instance outside of the class via the new operator    The magic method   clone   is declared as private to prevent cloning    of an instance of the class via the clone operator    The magic method   wakeup   is declared as private to prevent unserializing of an instance of the class via the global function   unserialize      A new instance is created via late static binding in the static creation    method getInstance   with the keyword static  This   allows the subclassing of the class Singleton in the example

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I agree with the first answer but I would also declare the class as final so that it cannot be extended as extending a singleton violates the singleton pattern   Also the instance variable should be private so that it cannot be accessed directly   Also make the   clone method private so that you cannot clone the singleton object   Below is some example code          Singleton class        final class UserFactory       private static   instance   null                  Private constructor                    private function   construct                      Private clone method                     private function   clone                      Call this method to get singleton                return UserFactory             public static function getInstance                 if  self    instance     null                self    instance   new UserFactory                      return self    instance            Example Usage   user factory   UserFactory  getInstance      What this stops you from doing  which would violate the singleton pattern      YOU CANNOT DO THIS    user factory   UserFactory    instance   class SecondUserFactory extends UserFactory

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The Real One and Modern way to make Singleton Pattern is    lt  php         Singleton Pattern         Modern implementation      class Singleton                  Call this method to get singleton             public static function instance               static  instance   false        if   instance     false                      Late static binding  PHP 5 3            instance   new static                   return  instance                        Make constructor private  so nobody can call  new Class               private function   construct                      Make clone magic method private  so nobody can clone instance              private function   clone                      Make sleep magic method private  so nobody can serialize instance              private function   sleep                      Make wakeup magic method private  so nobody can unserialize instance              private function   wakeup           So now you can use it like    lt  php         Database        Inherited from Singleton  so it s now got singleton behavior      class Database extends Singleton      protected  label              Example of that singleton is working correctly          public function setLabel  label           this- gt label    label         public function getLabel           return  this- gt label             create first instance  database   Database  instance     database- gt setLabel  Abraham    echo  database- gt getLabel     PHP EOL      now try to create other instance as well  other db   Database  instance    echo  other db- gt getLabel     PHP EOL     Abraham   other db- gt setLabel  Priler    echo  database- gt getLabel     PHP EOL     Priler echo  other db- gt getLabel     PHP EOL     Priler   As you see this realization is lot more flexible

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You probably should add a private   clone   method to disallow cloning of an instance   private function   clone        If you don t include this method the following gets possible   inst1 UserFactory  Instance       to stick with the example provided above  inst2 clone  inst1    now  inst1      inst2 - they are not the same instance any more

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You probably should add a private   clone   method to disallow cloning of an instance   private function   clone        If you don t include this method the following gets possible   inst1 UserFactory  Instance       to stick with the example provided above  inst2 clone  inst1    now  inst1      inst2 - they are not the same instance any more

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