常量
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魔术常量

PHP 向它运行的任何脚本提供了大量的预定义常量。不过很多常量都是由不同的扩展库定义的，只有在加载了这些扩展库时才会出现，或者动态加载后，或者在编译时已经包括进去了。

有七个魔术常量它们的值随着它们在代码中的位置改变而改变。例如 __LINE__ 的值就依赖于它在脚本中所处的行来决定。这些特殊的常量不区分大小写，如下：

几个 PHP 的“魔术常量”
名称 说明
__LINE__ 文件中的当前行号。
__FILE__ 文件的完整路径和文件名。如果用在被包含文件中，则返回被包含的文件名。自 PHP 4.0.2 起，__FILE__ 总是包含一个绝对路径（如果是符号连接，则是解析后的绝对路径），而在此之前的版本有时会包含一个相对路径。
__DIR__ 文件所在的目录。如果用在被包括文件中，则返回被包括的文件所在的目录。它等价于 dirname(__FILE__)。除非是根目录，否则目录中名不包括末尾的斜杠。（PHP 5.3.0中新增） =
__FUNCTION__ 函数名称（PHP 4.3.0 新加）。自 PHP 5 起本常量返回该函数被定义时的名字（区分大小写）。在 PHP 4 中该值总是小写字母的。
__CLASS__ 类的名称（PHP 4.3.0 新加）。自 PHP 5 起本常量返回该类被定义时的名字（区分大小写）。在 PHP 4 中该值总是小写字母的。
__METHOD__ 类的方法名（PHP 5.0.0 新加）。返回该方法被定义时的名字（区分大小写）。
__NAMESPACE__ 当前命名空间的名称（大小写敏感）。这个常量是在编译时定义的（PHP 5.3.0 新增）

参见 get_class()，get_object_vars()，file_exists() 和 function_exists()。

常量
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PHP手册 - N: 魔术常量

用户评论:

Devender Bejju (19-Mar-2012 11:17)

@warhog FYI: __halt_compiler is not a constant its a function

See signature

  void __halt_compiler ( void )

david at thegallagher dot net (22-Feb-2012 09:19)

You cannot check if a magic constant is defined. This means there is no point in checking if __DIR__ is defined then defining it. `defined('__DIR__')` always returns false. Defining __DIR__ will silently fail in PHP 5.3+. This could cause compatibility issues if your script includes other scripts.

Here is proof:

<pre><?php
echo (defined('__DIR__') ? '__DIR__ is defined' : '__DIR__ is NOT defined' . PHP_EOL);
echo (defined('__FILE__') ? '__FILE__ is defined' : '__FILE__ is NOT defined' . PHP_EOL);
echo (defined('PHP_VERSION') ? 'PHP_VERSION is defined' : 'PHP_VERSION is NOT defined') . PHP_EOL;
echo 'PHP Version: ' . PHP_VERSION . PHP_EOL;
?></pre>

Output:
__DIR__ is NOT defined
__FILE__ is NOT defined
PHP_VERSION is defined
PHP Version: 5.3.6

Anonymous (27-Dec-2011 04:44)

Further clarification on the __TRAIT__ magic constant.

<?php
trait PeanutButter {
    function traitName() {echo __TRAIT__;}
}

trait PeanutButterAndJelly {
    use PeanutButter;
}

class Test {
    use PeanutButterAndJelly;
}

(new Test)->traitName(); //PeanutButter
?>

jrivero24 at yahoo dot es (05-Aug-2011 10:37)

When __DIR__ is not defined, prior 5.3.0:

<?php if ( !defined('__DIR__') ) define('__DIR__', dirname(__FILE__)); ?>

user9 at voloreport dot com (10-Jul-2011 08:33)

Note that __FILE__ has a quirk when used inside an eval() call. It will tack on something like "(80) : eval()'d code" (the number may change) on the end of the string at run-time. The workaround is:

$script = php_strip_whitespace('myprogram.php');
$script = str_replace('__FILE__',"preg_replace('@\(.*\(.*$@', '', __FILE__,1)",$script);
eval($script);

chris dot kistner at gmail dot com (20-Apr-2011 01:16)

There is no way to implement a backwards compatible __DIR__ in versions prior to 5.3.0.

The only thing that you can do is to perform a recursive search and replace to dirname(__FILE__):
find . -type f -print0 | xargs -0 sed -i 's/__DIR__/dirname(__FILE__)/'

Jamie (02-Mar-2011 09:13)

Note that as mentioned, __FILE__ resolves any aliases. Other real path information, such as $_SERVER["SCRIPT_FILENAME"], doesn't.

__FILE__ => /volume1/web/mysite/admin/inc/includeFile.inc.php
$_SERVER["SCRIPT_FILENAME"] => /var/services/web/mysite/admin/products.php

If you need to compare one with the other, use
realpath($_SERVER["SCRIPT_FILENAME"])

stefan at efectos dot nl (08-Nov-2010 01:58)

When __DIR__ is not defined, you can also use this workaround to generate it:

<?php
if(!defined('__DIR__')) {
    $iPos = strrpos(__FILE__, "/");
    define("__DIR__", substr(__FILE__, 0, $iPos) . "/");
}
?>

Keep in mind this sets __DIR__ to the directory you are running this snippet from.

madboyka at yahoo dot com (10-Sep-2010 03:37)

Since namespace were introduced, it would be nice to have a magic constant or function (like get_class()) which would return the class name without the namespaces.

On windows I used basename(__CLASS__). (LOL)

Anonymous (08-Aug-2010 11:39)

__DIR__ befor PHP 5.3.0

<?php
if (!defined('__DIR__')) {
  class __FILE_CLASS__ {
    function  __toString() {
      $X = debug_backtrace();
      return dirname($X[1]['file']);
    }
  }
  define('__DIR__', new __FILE_CLASS__);
}
?>

me at jamessocol dot com (25-Jun-2008 05:11)

We need an eighth magic constant, something along the lines of __STATIC__. This should return the name of the class from which a static method was called, regardless of where in the inheritance tree the method was defined.

PHP 5.3 has the new use of the static keyword which will help, but it isn't perfect. You still have to repeat yourself frequently.

For example, trying to implement Active Record:

<?php

// In PHP 5.3

class Model
{
    public static function find()
    {
        echo static::$class;
    }
}

class Product extends Model
{
    protected static $class = __CLASS__;
}

class User extends Model
{
    protected static $class = __CLASS__;
}

Product::find(); // "Product"
User::find(); // "User"

?>

<?php

// With __STATIC__ keyword. (Would be better.)

class Model
{
    public static function find()
    {
        echo __STATIC__;
    }
}

class Product extends Model {}

class User extends Model {}

Product::find(); // "Product"
User::find(); // "User"

?>

[EDITED : Use get_called_class()]

php at kennel17 dot co dot uk (20-Jun-2007 06:29)

Further to my previous note, the 'object' element of the array can be used to get the parent object.  So changing the get_class_static() function to the following will make the code behave as expected:

<?php
    function get_class_static() {
        $bt = debug_backtrace();
   
        if (isset($bt[1]['object']))
            return get_class($bt[1]['object']);
        else
            return $bt[1]['class'];
    }
?>

HOWEVER, it still fails when being called statically.  Changing the last two lines of my previous example to

<?php
  foo::printClassName();
  bar::printClassName();
?>

...still gives the same problematic result in PHP5, but in this case the 'object' property is not set, so that technique is unavailable.

php at kennel17 dot co dot uk (20-Jun-2007 05:12)

In response to stangelanda at gmail dot com, (who suggested a possible fix to get the actual class name of the object, when being called statically).

in PHP5, this fix no longer works. 

Here is some example code:

<?php

  function get_class_static() {
    $bt = debug_backtrace();
    $name = $bt[1]['class'];
    return $name;
  }

  class foo {
    function printClassName() {
      print(get_class_static() . "<br>");
     }
  }

  class bar extends foo {
  }

$f = new foo();
$b = new bar();
$f->printClassName();
$b->printClassName();

?>

In PHP4, it outputs
  foo
  bar
as you described.

However, in PHP5, due to the way the debug_backtrace() function has been modified (see http://bugs.php.net/bug.php?id=30828) the output is now
  foo
  foo

I have yet to figure out a way to get the original output in PHP5.  Any suggestions would be very useful, and if I find an answer I'll post it here.

Tomek Perlak [tomekperlak at tlen pl] (10-Nov-2006 10:16)

The __CLASS__ magic constant nicely complements the get_class() function.

Sometimes you need to know both:
- name of the inherited class
- name of the class actually executed

Here's an example that shows the possible solution:

<?php

class base_class
{
    function say_a()
    {
        echo "'a' - said the " . __CLASS__ . "<br/>";
    }

    function say_b()
    {
        echo "'b' - said the " . get_class($this) . "<br/>";
    }

}

class derived_class extends base_class
{
    function say_a()
    {
        parent::say_a();
        echo "'a' - said the " . __CLASS__ . "<br/>";
    }

    function say_b()
    {
        parent::say_b();
        echo "'b' - said the " . get_class($this) . "<br/>";
    }
}

$obj_b = new derived_class();

$obj_b->say_a();
echo "<br/>";
$obj_b->say_b();

?>

The output should look roughly like this:

'a' - said the base_class
'a' - said the derived_class

'b' - said the derived_class
'b' - said the derived_class

stangelanda at gmail dot com (06-Sep-2006 05:17)

claude noted that __CLASS__ always contains the class that it is called in, if you would rather have the class that called the method use get_class($this) instead.  However this only works with instances, not when called statically.

<?php
  class A {
     function showclass() {
         echo get_class($this);
     }
  }

  class B extends A {}

  $a = new A();
  $b = new B();

  $a->showclass();
  $b->showclass();
  A::showclass();
  B::showclass();

  //results in "a", "b", false, false
?>

I tried keeping track of the class manually within the properties, but the following doesn't work either:

<?php
  class A {
     var $class = __CLASS__;
     function showclass() {
         echo $this->class;
     }
  }

  class B extends A {
     var $class = __CLASS__;
  }

  //results in "a", "b", NULL, NULL
?>

The best solution I could come up with was using debug_backtrace.  I assume there is a better way somehow, but I can't find it.  However the following works:

<?php
  class A {
     function showclass() {
        $backtrace = debug_backtrace();
        echo $backtrace[0]['class'];
     }
  }

  class B extends A {}

  //results in "a", "b", "a", "b"
?>

warhog at warhog dot net (18-Dec-2005 09:33)

There is another magic constant not mentioned above: __COMPILER_HALT_OFFSET__ - contains where the compiler halted - see http://www.php.net/manual/function.halt-compiler.php for further information.

vijaykoul_007 at rediffmail dot com (22-Sep-2005 05:59)

the difference between
__FUNCTION__ and __METHOD__ as in PHP 5.0.4 is that

__FUNCTION__ returns only the name of the function

while as __METHOD__ returns the name of the class alongwith the name of the function

class trick
{
      function doit()
      {
                echo __FUNCTION__;
      }
      function doitagain()
      {
                echo __METHOD__;
      }
}
$obj=new trick();
$obj->doit();
output will be ----  doit
$obj->doitagain();
output will be ----- trick::doitagain

karl __at__ streetlampsoftware__dot__com (03-Mar-2005 09:39)

Note that the magic constants cannot be included in quoted strings.

For instance,
echo "This is the filename: __FILE__";
will return exactly what's typed above.

echo "This is the filename: {__FILE__}";
will also return what's typed above.

The only way to get magic constants to parse in strings is to concatenate them into strings:
echo "This is the filename: ".__FILE__;

csaba at alum dot mit dot edu (03-Mar-2005 12:04)

Sometimes you might want to know whether a script is the top level script or whether it has been included.  That could be useful if you want to reuse the routines in another script, but you don't want to separate them out.  Here's a way that seems to be working for me (for both Apache2 module and CLI versions of PHP) on my Win XP Pro system.

By the way, if __FILE__ is within a function call, its value corresponds to the file it was defined in and not the file that it was called from.  Also, I used $script and strtolower instead of realpath because if the script is deleted after inclusion but before realpath is called (which could happen if the test is deferred), then realpath would return empty since it requires an extant file or directory.

Csaba Gabor from Vienna

<?php
if (amIincluded()) return;    // if we're included we only want function defs
function amIincluded() {
//    returns true/false depending on whether the currently
//    executing script is included or not
//    Don't put this function in an include file (duh)!
    $webP = !!$_SERVER['REQUEST_METHOD'];    // a web request?
    $script = preg_replace('/\//',DIRECTORY_SEPARATOR,
                           $_SERVER['SCRIPT_FILENAME']);
    return ($webP) ? (strtolower(__FILE__)!=strtolower($script)) :
           !array_key_exists("_REQUEST", $GLOBALS);
}
?>

lm arobase bible point ch (08-Dec-2004 10:17)

in reply to x123 at bestof dash inter:
I believe, this is not a bug, but a feature.
__FILE__ returns the name of the include file, while $PHP_SELF returns the relative name of the main file.
It is then easy to get the file name only with substr(strrchr($PHP_SELF,'/'),1)

claude at NOSPAM dot claude dot nl (18-Jul-2004 04:29)

Note that __CLASS__ contains the class it is called in; in lowercase. So the code:

class A
{
    function showclass()
    {
        echo __CLASS__;
    }
}

class B extends A
{
}

$a = new A();
$b = new B();

$a->showclass();
$b->showclass();
A::showclass();
B::showclass();

results in "aaaa";

ulrik (04-Mar-2004 03:44)

note that __FUNCTION__ define gives the the function name in lowercase

warhog at warhog dot net (06-Feb-2004 08:49)

just to read out the filename of the currently proceeded file use
<?php basename(__FILE__); ?>

hixon at colorado dot edu (16-May-2003 01:21)

You can use the following in files that you want to include, but not run directly.  The script will exit if it's run as the top-level script, but will not exit if it's included from another script.  Of course this won't work in the command line mode.

if (realpath(__FILE__) == realpath($_SERVER['SCRIPT_FILENAME'])) {
  exit;
}

kop at meme dot com (13-Feb-2003 11:34)

The keywords TRUE and FALSE (case insensitive), which represent their respective boolean values, are worth noting here.

darwin[at]buchner[dot]net (15-Mar-2002 12:54)

As of version 4.0.6, there is also a handy predefined DIRECTORY_SEPARATOR constant which you can use to make you scripts more portatable between OS's with different directory structures.