class_exists

说明

bool class_exists ( string class_name [, bool autoload] )

如果由 class_name 所指的类已经定义，此函数返回 TRUE，否则返回 FALSE。

例 1. class_exists() 例子


<?php
// Check the class exists before trying to use it
if (class_exists('MyClass')) {
    $myclass = new MyClass();
}

?>

class_exists() 默认将会尝试调用 __autoload，如果不想让 class_exists() 调用 __autoload，可以将 autoload 参数设为 FALSE。

例 2. autoload 参数例子


<?php
function __autoload($class)
{
    include($class . '.php');

    // Check to see if the include declared the class
    if (!class_exists($class, false)) {
        trigger_error("Unable to load class: $class", E_USER_WARNING);
    }
}

if (class_exists('MyClass')) {
    $myclass = new MyClass();
}

?>

注意: autoload 参数是 PHP 5 添加的。

参见 interface_exists() 和 get_declared_classes()。

add a note User Contributed Notes

pogrebnoj dot konstantin at gmail dot com
10-Aug-2007 01:39


What about the second argument of this function?



class_exists(string class, [bool autoload]).



class_exists() will attempt to call __autoload by default, if you don't want class_exists() to call __autoload, you can set the parameter autoload  to FALSE.



The autoload parameter was added in PHP 5

peterw
13-May-2007 11:27


Using require_once() would help against including a class file more than once.

Frayja
01-Jun-2006 08:42


Like someone else pointed out class_exists() is case-INsensitive. 



Using in_array() which is case-sensitive, the following function is a case-sensitive version of class_exists().



<?php

function class_exists_sensitive( $classname )

{

   return ( class_exists( $classname ) && in_array( $classname, get_declared_classes() ) );

}

?>

josh at digitalfruition dot com
25-Jun-2005 06:23


Just to note, class names appear to be case INsensitive. (At least at the time of writing in PHP 4.3.9). Take the following example:



<?php



class Foo { var $myVar; }



class Foo_Bar extends Foo { var $myVar2;}



echo class_exists('Foo'); //true

echo class_exists('foo'); //true

echo class_exists('Foo_Bar'); // true

echo get_class_parent('Foo_Bar'); // foo (NOTE: NOT Foo!)



?>

andrey at php dot net
24-Oct-2004 11:43


In 5.0.0 and 5.0.1 there was no interface_exists() and in these versions class_exists() used to return TRUE for known interfaces. Starting 5.0.2 this is no more.

06-Apr-2004 12:04


Just a note that at least PHP 4.3.1 seems to crash under some situations if you call class_exists($foo) where $foo is an array (that is, the calling code is incorrect but the error recovery is far from perfect).

anonymous at somewhere dot tld
17-Jul-2003 07:20


If you have a directory of classes you want to create. (Modules in my instance)... you can do it like that





<?php


if (is_dir($this->MODULE_PATH) && $dh = opendir($this->MODULE_PATH)) {


   while (($file = readdir($dh)) !== false) {        


      if (preg_match("/(Mod[a-zA-Z0-9]+).php/", $file, $matches)>0) {                


         // include and create the class               


         require_once($this->MODULE_PATH."/".$file);


         $modules[] = new $matches[1]();


      }                


   }


} else {


   exit;


}


?>





//---


Here the rule is that all modules are on the form


ModModulename.php and that the class has the same name as the file.


The $modules array has all the classes initialized after this code

cristiano at aspatech dot com dot br
24-Jun-2002 06:36


This can be veeeery usefull if you use classes that uses other classes, which can be used in your front end. In other words, when you lost the control of which classes are declared in which point of the application, that can generate the "Cannot redeclare class". Use like





<?php


if ( !class_exists( "YourClass" ) ) {





    class YourClass {


        //your code


    }


}


?>





Thats it... Resolve all your problems =)

spamless_blair at nb dot net
09-Oct-2001 08:48


I have a script that includes various class libraries depending on what is contained in the constant _INCLUDE_LIST which is a comma-delimited string.





define('_INCLUDE_LIST', 'CORE, LIB_DATA, LIB_EMAIL');





I use class_exists() to determine if a class definition has been included before creating an instance of it.





if(class_exists('CMySQLConnection')) $oData = new CMySQLConnection;





Hope it is helpful for someone!





-Jason Garber


IonZoft.com

emurano at optushome dot com dot au
09-Mar-2001 08:35


Multiple inclusion protection can be achieved in php3 and for non class related code, for example:





<?php


if(!defined("_SOME_UNIQUE_CONSTANT_")) {


define("_SOME_UNIQUE_CONSTANT_",TRUE);





// Your protected code here





}


?>





Make sure that you use a different constant for each protected piece of code.  Generally this is used on a file per file basis. Just #ifndef in C!

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