Difference between require include require once and include once

Question

In PHP    When should I use require vs  include  When should I use require once vs  include once

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Include   Require you can include the same file more than once also      require   is identical to include   except upon failure it will also produce a fatal E COMPILE ERROR level error  In other words  it will halt the script whereas include   only emits a warning  E WARNING  which allows the script to continue    require once   include once     is identical to include require except PHP will check if the file has already been included  and if so  not include  require  it again

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Require critical parts  like authorization and include all others   Multiple includes are just very bad design and must be avoided at all  So    once doesn t really matter

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With require the file must exist  if it doesn t then an error will display  whereas with include - if the file doesn t exist then then the page will continue loading

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nbsp    When should one use require or include   The require and include functions do the same task  i e  includes and evaluates the specified file  but the difference is require will cause a fatal error when the specified file location is invalid or for any error whereas include will generate a warning and continue the code execution   So you may use the require function in the case where the file you are trying to include is the heart of the system and may cause a huge impact on rest of the code and you can use the include function when the file you are trying to include is a simple file containing some less important code   And my personal recommendation  for less important code  is to go for the require function everywhere in your code while it is in development phase such that you can debug code and later on replace all require functions by include function before moving it to production such that if you miss any bugs it will not affect the end user and the rest of the code executes properly    When should one use require once or require   The basic difference between require and require once is require once will check whether the file is already included or not if it is already included then it won t include the file whereas the require function will include the file irrespective of whether file is already included or not   So in cases where you want to include some piece of code again and again use the require function whereas if you want to include some code only once in your code  use require once

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include   will throw a warning if it can t include the file  but the rest of the script will run   require   will throw an E COMPILE ERROR and halt the script if it can t include the file   The include once   and require once   functions will not include the file a second time if it has already been included   See the following documentation pages    include require include once require once

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include   will generate a warning when it doesn t find a file  but require once   will generate a fatal error   Another thing is if file is included before  Then require once   will not include it again

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Basically  if you require a wrong path  PHP throws a fatal error and the shutdown function is called  but when you include a wrong path  PHP will continue execution  but it will just display a warning that the file does not exist   From the English word require  PHP is told that the execution of the page or file depends on the file required   From my experience  it s norm to require important files such as configuration files  database classes and other important utilities

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Use  include  for reusable PHP templates  Use  require  for required libraries      once  is nice  because it checks whether the file is already loaded or not  but it only makes sense for me in  require once

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Use require function when you need to load any class  function  or dependency  Use include function when you want to load template-styled file   If you are still confused  just use require once for all time

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It s often a matter of whether you want to load a client library conditionally  or go ahead and load it whether or not you re going to use it   Here s concrete example  elaborating on what pcj said   Say you have a configuration file storing your database username and password  conf php     lt  php   my site configuration file    For Database  location  localhost    dbuser  yourname    userpw  yourpassword    database  nameofdatabase     gt    And a class with a static function that uses the database    lt  php class UsedInLoop       public static function databaseQuery            require  path to conf php                    require once will not work here          db   new mysqli  location   dbuser   userpw   database             yada yada yada           gt    And that static function is used inside of another function that is being called iteratively inside a loop    lt  php require once  path to arbitraryObject php       either will likely be OK at this level  obj   new arbitraryObject    foreach  array as  element        obj- gt myFunction        gt    You can only require include the class once   If you require include it on every iteration of your loop  you ll get an error   However  you have to include your conf file every time the static function is called    lt  php class arbitraryObject       public function myFunction            require once  path to UsedInLoop php       This must be require once  require   will not work         UsedInLoop  databaseQuery              gt    Of course  moving it outside of the function could be a solution to that problem    lt  php require  path to UsedInLoop php       now require   is fine    class arbitraryObject       public function myFunction            UsedInLoop  databaseQuery              gt    Unless you re concerned with the overhead of loading a class that might only be used in certain conditions and don t want to load it when it isn t

User · Answer

There are require and include once as well   So your question should be       When should I use require vs  include  When should I use require once vs  require   The answer to 1 is described here      The require   function is identical to include    except that it handles errors differently  If an error occurs  the include   function generates a warning  but the script will continue execution  The require   generates a fatal error  and the script will stop    The answer to 2 can be found here      The require once   statement is identical to require   except PHP will check if the file has already been included  and if so  not include  require  it again

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require has greater overhead than include  since it has to parse the file first  Replacing requires with includes is often a good optimization technique

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From the manual      require   is identical to include   except upon failure it will also produce a fatal E COMPILE ERROR level error  In other words  it will halt the script whereas include   only emits a warning  E WARNING  which allows the script to continue    The same is true for the  once   variants

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Whenever you are using require once   can be use in a file to include another file when you need the called file only a single time in the current file   Here in the example I have an test1 php    lt  php   echo  today is   date  Y-m-d        gt      and in another file that I have named test2 php   lt  php   require once  test1 php      require once  test1 php        gt    as you are watching the m requiring the the test1 file twice but the file will include the test1 once and for calling at the second time this will be ignored   And without halting will display the output a single time   Whenever you are using  include once    can be used in a file to include another file when you need the called file more than once in the current file   Here in the example I have a file named test3 php    lt  php   echo  today is   date  Y-m-d        gt     And in another file that I have named test4 php   lt  php   include once  test3 php      include once  test3 php        gt    as you are watching the m including the test3 file will include the file a single time but halt the further execution

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Just use require and include   Because think how to work with include once or require once  That is looking for log data which save included or required PHP files  So that is slower than include and require   if   defined php         include  php       define php  1       Just using like this

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You should keep class and function definitions organized in files   Use require once   to load dependencies  classes  functions  constants    Use require   to load template-like files   Use include once   to load optional dependencies  classes  functions  constants    Use include   to load optional template-like files

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An answer after 7 years for 2018 This question was asked seven years ago  and none of answers provide  practical help for the question  In the modern PHP programming you mainly use required once only once to include your autoloader class  composer autoloader often   and it will loads all of your classes and functions  functions files need to be explicitly added to composer json file to be available in all other files   If for any reason your class is not loadable from autoloader you use require once to load it  There are some occasions that we need to use require  For example  if you have a really big array definition and you don t want to add this to your class definition source code you can    lt  php    arr php return   x   gt  y       lt  php   main php  arr  require  arry php   If the file that you intend to include contains something executable or declares some variables you almost always need to use require  because if you use require once apart from the first place your code will not be executed and or your variables will not initiate silently  causing bugs that are absolutely hard to pinpoint  There is no practical use case for include and include once really

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My suggestion is to just use require once 99 9  of the time   Using require or include instead implies that your code is not reusable elsewhere  i e  that the scripts you re pulling in actually execute code instead of making available a class or some function libraries     If you are require including code that executes on the spot  that s procedural code  and you need to get to know a new paradigm   Like object oriented programming  function-based programming  or functional programming   If you re already doing OO or functional programming  using include once is mostly going to be delaying where in the stack you find bugs errors   Do you want to know that the function do cool stuff   is not available when you go to call for it later  or the moment that you expect it to be available by requiring the library   Generally  it s best to know immediately if something you need and expect isn t available  so just use require once   Alternatively  in modern OOP  just autoload your classes upon use

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I was using function as below   function doSomething         require once xyz php                There were constant values declared in xyz php   I have to call this doSomething   function from another PHP script file   But I observed behavior while calling this function in a loop  for first iteration doSomething   was getting constant values within xyz php  but later every iteration doSomething   was not able to get the constant values declared in xyz php   I solved my problem by switching from require once   to include    updated doSomething   code is as below   function doSomething         include xyz php                Now every iteration call to doSomething   gets the constant values defined in xyz php

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one thing I noticed  when using include I can only access the included files functions from the file that included it  With require once  I can run that function in a second required once file    also  I recommend adding   if file exists  RequiredFile        require once  RequiredFile    else    die  Error  File Does Not Exist        Because when require once kills the page  it can sometimes echo the directory of your website files  Here s a custom function I made to require files    function addFile  file   type    php    important false         site-content is a directory where I store all the files that I plan to require once       the site-content directory has  deny from all  in its  htaccess file to block direct connections     if  type  amp  amp  file exists  site-content    file      type   amp  amp   is dir  site-content    file      type               is dir checks that the file is not a folder         require once  site-content    file      type           return  site-content    file      type       else if   type  amp  amp  file exists  site-content    file   amp  amp   is dir  site-content    file              if you set   type false  you can add the file type   php   ect  to the end of the   file   useful for requiring files named after changing vars          require once  site-content    file           return  site-content    file       else if  important             if you set  important to true  the function will kill the page  which also prevents accidentally echoing the main directory path of the server          die  Server Error  Files Missing            return false       else            the function returns false if the file does not exist  so you can check if your functions were successfully added         return false            usage example    success   addFile  functions common     if  success       commonFunction     else      fallbackFunction

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Use   require when the file is required by your application  e g  an important message template or a file containing configuration variables without which the app would break  require once when the file contains content that would produce an error on subsequent inclusion  e g   function important        important code     is definitely needed in your application but since functions cannot be redeclared should not be included again  include when the file is not required and application flow should continue when not found  e g  great for templates referencing variables from the current scope or something include once optional dependencies that would produce errors on subsequent loading or maybe remote file inclusion that you do not want to happen twice due to the HTTP overhead   But basically  it s up to you when to use which

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Difference between  once functions and without  once functions  without  once code will be included again whereas with  once functions PHP keeps track of the included files and will include it only once   Difference between require and include  If a required file is not found PHP will emit a fatal error whereas for include only a warning will be emitted

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They are all ways of including files   Require means it needs it  Require once means it will need it but only requires it once  Include means it will include a file but it doesn   t need it to continue   Examples   Require  filename  Require once  filename  Include  filename    There is also an include once function which includes a file once   Include once  filename    Don   t use capital letters where I have as I am typing from my phone

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In the age of PSR-0   PSR-4 autoloaders  it may be completely unnecessary to use any of the statements if all you need is to make some functions   classes available to your code  of course  you still need to require once autoloader itself in your bootstrap file and include templates if you still use PHP as a template engine

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The difference is in the error the commands generate  With require  the file you want to use is really required and thus generates an E ERROR if it is not found       require   is identical to include    except upon failure it will also produce a fatal E ERROR  level error    include only generates an E WARNING error if it fails which is more or less silent   So use it if the file is required to make the remaining code work and you want the script to fail the file is not available     For   once        include once   may be used in cases where the same file might be included and evaluated more than once during a particular execution of a script  so in this case it may help avoid problems such as function redefinitions  variable value reassignments  etc    Same applies to require once   of course     Reference  require    include once

[php] Difference between require, include, require_once and include_once?

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