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  • define()

    (PHP 4, PHP 5, PHP 7)

    定义一个常量

    说明

    define(string $name,mixed $value[,bool $case_insensitive= false]): bool

    在运行时定义一个常量。

    参数

    $name

    常量名。

    $value

    常量的值;在 PHP 5 中,$value必须是标量(integer、float、string、boolean、NULL)在 PHP 7 中还允许是个array的值。

    Warning

    常量还可以定义为resource类型,但并不推荐这样做,因为可能会有不可预知的行为发生。

    $case_insensitive

    如果设置为TRUE,该常量则大小写不敏感。默认是大小写敏感的。比如,CONSTANTConstant代表了不同的值。

    Note:

    大小写不敏感的常量以小写的方式储存。

    返回值

    成功时返回TRUE,或者在失败时返回FALSE

    更新日志

    版本说明
    7.0.0允许array的值。

    范例

    定义常量

    <?php
    define("CONSTANT", "Hello world.");
    echo CONSTANT; // 输出 "Hello world."
    echo Constant; // 输出 "Constant" 并导致 Notice
    define("GREETING", "Hello you.", true);
    echo GREETING; // 输出 "Hello you."
    echo Greeting; // 输出 "Hello you."
    //  PHP 7 起就可以运行了
    define('ANIMALS', array(
        'dog',
        'cat',
        'bird'
    ));
    echo ANIMALS[1]; // 输出 "cat"
    ?>
    

    参见

    • defined() 检查某个名称的常量是否存在
    • constant() 返回一个常量的值
    • Constants这一节
    Be aware that if "Notice"-level error reporting is turned off, then trying to use a constant as a variable will result in it being interpreted as a string, if it has not been defined.
    I was working on a program which included a config file which contained:
    <?php
    define('ENABLE_UPLOADS', true);
    ?>
    Since I wanted to remove the ability for uploads, I changed the file to read:
    <?php
    //define('ENABLE_UPLOADS', true);
    ?>
    However, to my surprise, the program was still allowing uploads. Digging deeper into the code, I discovered this:
    <?php
    if ( ENABLE_UPLOADS ):
    ?>
    Since 'ENABLE_UPLOADS' was not defined as a constant, PHP was interpreting its use as a string constant, which of course evaluates as True.
    You can define constants with variable names (works also with constant values or variables or array values or class properties and so on - as long it's a valid constant name).
    <?php
      
      # Define a constant and set a valid constant name as string value
      define("SOME_CONSTANT", "NEW_CONSTANT");
      
      # Define a second constant with dynamic name (the value from SOME_CONSTANT)
      define(SOME_CONSTANT, "Some value");
      
      # Output
      echo SOME_CONSTANT; // prints "NEW_CONSTANT"
      echo "<br>";
      echo NEW_CONSTANT; // prints "Some value"
      
      ?>
    Needless to say that you'll lose your IDE support for refactoring and highlighting completely for such cases.
    No clue why someone would / could actually use this but i thought it's worth mentioning.
    define() will define constants exactly as specified. So, if you want to define a constant in a namespace, you will need to specify the namespace in your call to define(), even if you're calling define() from within a namespace. The following examples will make it clear.
    The following code will define the constant "MESSAGE" in the global namespace (i.e. "\MESSAGE").
    <?php
    namespace test;
    define('MESSAGE', 'Hello world!');
    ?>
    The following code will define two constants in the "test" namespace.
    <?php
    namespace test;
    define('test\HELLO', 'Hello world!');
    define(__NAMESPACE__ . '\GOODBYE', 'Goodbye cruel world!');
    ?>
    
    Not sure why the docs omit this, but when attempting to define() a constant that has already been defined, it will fail, trigger an E_NOTICE and the constant's value will remain as it was originally defined (with the new value ignored).
    (Guess that's why they're called "constants".)
    The value of a constant can be the value of another constant.
    <?php
    define("NEW_GOOD_NAME_CONSTANT", "I have a value");
    define("OLD_BAD_NAME_CONSTANT", NEW_GOOD_NAME_CONSTANT);
    echo NEW_GOOD_NAME_CONSTANT; // current
    echo OLD_BAD_NAME_CONSTANT; // legacy
    ?>
    
    This is obvious, but easy to forget: if you include a file, the include file can only make use of constants already defined. For example:
    <?php
    define("VEG","cabbage");
    require("another file");
    define("FRUIT","apple");
    // "another file":
    echo VEG; // cabbage
    echo FRUIT; // FRUIT
    ?>
    
    There's an undocumented side-effect of setting the third parameter to true (case-insensitive constants): these constants can actually be "redefined" as case-sensitive, unless it's all lowercase (which you shouldn't define anyway).
    The fact is that case-sensitive constants are stored as is, while case-insensitive constants are stored in lowercase, internally. You're still allowed to define other constants with the same name but capitalized differently (except for all lowercase).
    <?php
     // "echo CONST" prints 1, same as "echo const", "echo CoNst", etc.
     define('CONST', 1, true);
     echo CONST; // Prints 1
     define('CONST', 2);
     echo CONST; // Prints 2
     echo CoNsT; // Prints 1
     echo const; // Prints 1
     // ** PHP NOTICE: Constant const already defined ** 
     define('const', 3);
     echo const; // Prints 1
     echo CONST; // Prints 2
    ?>
    Why would you use this?
    A third party plugin might attempt to define a constant for which you already set a value. If it's fine for them to set the new value, assuming you cannot edit the plugin, you could define your constant case-insensitive. You can still access the original value, if needed, by using any capitalization other than the one the plugin uses. As a matter of fact, I can't think of another case where you would want a case-insensitive constant...
    Found something interesting. The following define:
    <?php
    define("THIS-IS-A-TEST","This is a test");
    echo THIS-IS-A-TEST;
    ?>
    Will return a '0'.
    Whereas this:
    <?php
    define("THIS_IS_A_TEST","This is a test");
    echo THIS_IS_A_TEST;
    ?>
    Will return 'This is a test'.
    This may be common knowledge but I only found out a few minutes ago.
    [EDIT BY danbrown AT php DOT net: The original poster is referring to the hyphens versus underscores. Hyphens do not work in defines or variables, which is expected behavior.]
    Php 7 - Define: "Defines a named constant at runtime. In PHP 7, array values are also accepted."
    But prior PHP 7, you can maybe do this, to pass an array elsewhere using define:
    $to_define_array = serialize($array);
    define( "DEFINEANARRAY", $to_define_array );
    ... and so ...
    $serialized = DEFINEANARRAY; // passing directly the defined will not work
       $our_array = unserialize($serialized); 
    print_r($our_array);
    I think worth mentioning is that define() appears to ignore invalid constant names.
    One immediate implication of this seem to be that if you use an invalid constant name you have to use constant() to access it and obviously that you can't use the return value from define() to tell you whether the constant name used is invalid or not.
    For example:
    $name = '7(/!§%';
    var_dump(define($name, "hello")); // outputs bool(true)
    var_dump(constant($name)); // outputs string(5) "hello"
    It may be worth stating that a define function must be executed before its global constant is referenced.
    Example:
    Abc();
    define("TEST", 23);
    function Abc()
      {
      echo TEST;
      } // Abc
    This code fails with a Notice-level message. TEST is treated here as being the string "TEST".
    For translating with variables and define, take also a look on the constant() function.
    Example : 
    <?php 
    define('PAYMENT_IDEAL',"iDEAL Payment ( NL only )");
    define('PAYMENT_MASTERCARD',"Mastercard Payment ( international )");
    $payparam='MASTERCARD';
    echo constant("PAYMENT_$payparam");
    // output : 
    // Mastercard Payment ( international )
    ?>
    
    To clear up a few thing:
    Integers with 0 in front work. But since PHP (and many other languages) handle them as octal values, they're only allowed a range of 0-7:
    <?php
     define('GOOD_OCTAL', 0700);
     define('BAD_OCTAL', 0800);
     print GOOD_OCTAL;
     print '<br>';
     print BAD_OCTAL;
    ?>
    Result:
    448
    0
    ---------------------------------------
    Furthermore, 
    writing the constant name without the quotation-marks (as mentioned in the notes) throws an E_NOTICE and should be avoided!
    <?php
     define(TEST, 'Throws an E_NOTICE');
    ?>
    Result:
    Notice: Use of undefined constant TEST - assumed 'TEST'
    With php 7 you can now define arrays. 
    consider the following code:
    <?php
    define( "EXPLENATIVES", [1 => "Foo Bar", 2 => "Fehw Bahr", 3 => "Foo Bahr", 4 => "Fooh Bar", 5 => "Fooh Bhar", 6 => "Foo Barr", 7 => "Foogh Bar", 8 => "Fehw Barr", 9 => "Fu bar", 10 => "Foo Bahr", 11 => "Phoo Bar", 12 => "Foo Bawr", 13 => "Phooh Baughr", 14 => "Foogan Bargan", 15 => "Foo Bahre", 16 => "Fu Bahar", 17 => "Fugh Bar", 18 => "Phou Baughr"]);
    //set up define methods using mixed values; both array and non-array values
    define("NAVBTNS", [EXPLENATIVES, "Nouns", "Verbs", "Adjectives"]);
    //function to create a dropdown menu using the EXPLENATIVES array $btn=EXPLENATIVES=assoc_array
    function construct_navbar_buttons(){
      
      $btns = '<ul class="nav navbar-nav">';
                
      foreach(NAVBTNS as $button => $btn){
        if(is_array($btn)){
          $btns .= '<li class="dropdown">
                <a class="dropdown-toggle" data-toggle="dropdown" href="?id='.$btn.'">
                  <i class="glyphicon glyphicon-user"></i> You Dare Say? <i class="glyphicon glyphicon-collapse-down"></i>
                </a>
                <ul class="dropdown-menu dropdown-user">';
          foreach(EXPLENATIVES as $key => $button){
            $btns .= '<li><a href="#">'.$button.'</a></li>';
          }
          $btns .= '</ul>';
        }else{  
          $btns .= '<li><a href="#">'.$btn.'</a></li>';
        }
      }
      
      $btns .= '</ul>';
      return $btns;
    }      
    Love this new implementation!
    ?>
    
    You can also define associative array as constant:
    <?php
    define("DB", [
      "DB_HOST_NAME" => "localhost",
      "DB_USER_NAME" => "root",
      "DB_PASSWORD" => "password",
      "DB_DATABASE" => "db_example"
    ]);
    prin_r(DB);
    Array ( [DB_HOST_NAME] => localhost [DB_USER_NAME] => root [DB_PASSWORD] => password [DB_DATABASE] => db_example )
    Some people like to have an ambiguous, engine-agnostic database include specified by assigning a single config variable to one of a series of constants. Unfortunately, this can easily become needlessly clunky if these defines are handled in an included config.php file, since more than one hook will throw an ugly "already defined" error.
    Here's a simple way to accomplish this architectural model without having to create a bunch of clumsy sanity checks that compromise scalability:
    <?php
    /* Available databases. --Kris */
    $dbengines = array();
    $dbengines[] = "mysql";
    $dbengines[] = "mysqli";
    $dbengines[] = "pgsql";
    foreach ( $dbengines as $engine )
    {
      if ( !defined( "_" . strtoupper( $engine ) . "_" ) )
      {
        define( "_" . strtoupper( $engine ) . "_", $engine );
      }
    }
    /* Which database engine shall we use? --Kris */
    $sql_type = _MYSQLI_;
    ?>
    Then, somewhere later in the stack....
    <?php
    /* Include the file with the desired sql class. --Kris */
    require_once( $sql_type . ".php" );
    /* Now you can use the same code for any supported db engine. --Kris */
    $sql = new sql();
    $res = $sql->query( .... );
    In PHP 5 it seems it was possible to redefine predefined constants under certain circumstances. The following code will output 42 in PHP 5:
    <?php
    namespace NS;
    define('TRUE', 42, false);
    echo TRUE; 
    ?>
    In PHP 7 the output is different (1).

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