define vs const

Question

In PHP  when do you use   define  FOO   1     and when do you use  const FOO   1       What are the main differences between those two

User · Accepted Answer

As of PHP 5 3 there are two ways to define constants  Either using the const keyword or using the define   function   const FOO    BAR   define  FOO    BAR      The fundamental difference between those two ways is that const defines constants at compile time  whereas define defines them at run time  This causes most of const s disadvantages  Some disadvantages of const are    const cannot be used to conditionally define constants  To define a global constant  it has to be used in the outermost scope   if             const FOO    BAR         Invalid      but if             define  FOO    BAR       Valid     Why would you want to do that anyway  One common application is to check whether the constant is already defined   if   defined  FOO          define  FOO    BAR       const accepts a static scalar  number  string or other constant like true  false  null    FILE     whereas define   takes any expression  Since PHP 5 6 constant expressions are allowed in const as well   const BIT 5   1  lt  lt  5        Valid since PHP 5 6 and invalid previously define  BIT 5   1  lt  lt  5      Always valid  const takes a plain constant name  whereas define   accepts any expression as name  This allows to do things like this   for   i   0   i  lt  32     i        define  BIT      i  1  lt  lt   i      consts are always case sensitive  whereas define   allows you to define case insensitive constants by passing true as the third argument  Note  defining case-insensitive constants is deprecated as of PHP 7 3 0     define  FOO    BAR   true   echo FOO     BAR echo foo     BAR    So  that was the bad side of things  Now let s look at the reason why I personally always use const unless one of the above situations occurs    const simply reads nicer  It s a language construct instead of a function and also is consistent with how you define constants in classes  const  being a language construct  can be statically analysed by automated tooling  const defines a constant in the current namespace  while define   has to be passed the full namespace name   namespace A B C     To define the constant A B C FOO  const FOO    BAR   define  A B C FOO    BAR     Since PHP 5 6 const constants can also be arrays  while define   does not support arrays yet  However  arrays will be supported for both cases in PHP 7   const FOO    1  2  3         Valid in PHP 5 6 define  FOO    1  2  3       Invalid in PHP 5 6 and valid in PHP 7 0    Finally  note that const can also be used within a class or interface to define a class constant or interface constant  define cannot be used for this purpose   class Foo       const BAR   2     Valid      But class Baz       define  QUX   2      Invalid     Summary  Unless you need any type of conditional or expressional definition  use consts instead of define  s - simply for the sake of readability

User · Answer

Until PHP 5 3  const could not be used in the global scope  You could only use this from within a class  This should be used when you want to set some kind of constant option or setting that pertains to that class  Or maybe you want to create some kind of enum   define can be used for the same purpose  but it can only be used in the global scope  It should only be used for global settings that affect the entire application   An example of good const usage is to get rid of magic numbers  Take a look at PDO s constants  When you need to specify a fetch type  you would type PDO  FETCH ASSOC  for example  If consts were not used  you d end up typing something like 35  or whatever FETCH ASSOC is defined as   This makes no sense to the reader   An example of good define usage is maybe specifying your application s root path or a library s version number

User · Answer

Most of these answers are wrong or are only telling half the story    You can scope your constants by using namespaces  You can use the  const  keyword outside of class definitions  However  just like in classes the values assigned using the  const  keyword must be constant expressions    For example   const AWESOME    Bob      Valid   Bad example   const AWESOME   whatIsMyName       Invalid  Function call  const WEAKNESS   4 5 6     Invalid  Arithmetic   const FOO   BAR   OF   SOAP     Invalid  Concatenation    To create variable constants use define   like so   define  AWESOME   whatIsMyName        Valid define  WEAKNESS   4   5   6      Valid define  FOO   BAR   OF   SOAP      Valid

User · Answer

NikiC s answer is the best  but let me add a non-obvious caveat when using namespaces so you don t get caught with unexpected behavior  The thing to remember is that defines are always in the global namespace unless you explicitly add the namespace as part of the define identifier  What isn t obvious about that is that the namespaced identifier trumps the global identifier  So     lt  php namespace foo        Note  when referenced in this file or namespace  the const masks the defined version      this may not be what you want expect   const BAR    cheers     define  BAR    wonka       printf  What kind of bar is a  s bar  n   BAR         To get to the define in the global namespace you need to explicitely reference it   printf  What kind of bar is a  s bar  n    BAR      namespace foo2        But now in another namespace  like in the default  the same syntax calls up the       the defined version    printf  Willy  s n   BAR     printf  three  s n    foo BAR         gt    produces   What kind of bar is a cheers bar   What kind of bar is a wonka bar  willy wonka  three cheers   Which to me makes the whole const notion needlessly confusing since the idea of a const in dozens of other languages is that it is always the same wherever you are in your code  and PHP doesn t really guarantee that

User · Answer

No one says anything about php-doc  but for me that is also a very significant argument for the preference of const          My foo-bar const     var string     const FOO    BAR

User · Answer

I know this is already answered  but none of the current answers make any mention of namespacing and how it affects constants and defines    As of PHP 5 3  consts and defines are similar in most respects   There are still  however  some important differences     Consts cannot be defined from an expression   const FOO   4   3  doesn t work  but define  CONST   4   3   does    The name passed to define must include the namespace to be defined within that namespace    The code below should illustrate the differences    namespace foo        const BAR   1      define  BAZ   2       define   NAMESPACE        BAZ   3      namespace       var dump get defined constants true        The content of the user sub-array will be   foo  BAR    gt  1   BAZ    gt  2   foo  BAZ    gt  3        UPDATE      The upcoming PHP 5 6 will allow a bit more flexibility with const   You will now be able to define consts in terms of expressions  provided that those expressions are made up of other consts or of literals   This means the following should be valid as of 5 6   const FOOBAR    foo      bar   const FORTY TWO   6   9     For future editors  THIS IS DELIBERATE  Read the answer comments below for more details const ULTIMATE ANSWER    The ultimate answer to life  the universe and everything is     FORTY TWO    You still won t be able to define consts in terms of variables or function returns though  so   const RND   mt rand    const CONSTVAR    var    will still be out

User · Answer

I believe that as of PHP 5 3  you can use const outside of classes  as shown here in the second example    http   www php net manual en language constants syntax php   lt  php    Works as of PHP 5 3 0 const CONSTANT    Hello World    echo CONSTANT    gt

User · Answer

define I use for global constants   const I use for class constants   You cannot define into class scope  and with const you can  Needless to say  you cannot use const outside class scope   Also  with const  it actually becomes a member of the class  and with define  it will be pushed to global scope

User · Answer

To add on NikiC s answer  const can be used within classes in the following manner   class Foo       const BAR   1       public function myMethod             return self  BAR            You can not do this with define

User · Answer

Yes  const are defined at compile-time and as nikic states cannot be assigned an expression  as define   s can  But also const s cannot be conditionally declared  for the same reason   ie  You cannot do this   if     some condition         const WHIZZ   true      CANNOT DO THIS      Whereas you could with a define    So  it doesn t really come down to personal preference  there is a correct and a wrong way to use both   As an aside    I would like to see some kind of class const that can be assigned an expression  a sort of define   that can be isolated to classes

User · Answer

With define keyword constant  you will get the facilities of case insensitive but with const keyword you did not   define  FOO   1  true   echo foo    1 echo   lt br  gt    echo FOO    1 echo   lt br  gt     class A     const FOO   1     echo A  FOO    valid echo   lt br  gt       but  class B     define FOO   1    syntax error  unexpected  define     echo B  FOO    invalid

[php] define() vs. const

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