class_exists()

(PHP 4, PHP 5, PHP 7)

检查类是否已定义

说明

class_exists(string $class_name[,bool $autoload= true]): bool

检查指定的类是否已定义。

参数

$class_name

类名。名字的匹配是不分区大小写的。

$autoload

是否默认调用__autoload。

返回值

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

更新日志

范例

Example #1class_exists()例子

<?php
// 使用前检查类是否存在
if (class_exists('MyClass')) {
    $myclass = new MyClass();
}
?>

Example #2$autoloadparameter 例子

<?php
function __autoload($class)
{
    include($class . '.php');
    // Check to see whether 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();
}
?>

参见

• function_exists() 如果给定的函数已经被定义就返回 TRUE
• interface_exists() 检查接口是否已被定义
• get_declared_classes() 返回由已定义类的名字所组成的数组

If you are using aliasing to import namespaced classes, take care that class_exists will not work using the short, aliased class name - apparently whenever a class name is used as string, only the full-namespace version can be used
use a\namespaced\classname as coolclass;
class_exists( 'coolclass' ) => false

If you recursively load several classes inside an autoload function (or mix manual loading and autoloading), be aware that class_exists() (as well as get_declared_classes()) does not know about classes previously loaded during the *current* autoload invocation.
Apparently, the internal list of declared classes is only updated after the autoload function is completed.

Beware: class_exists is case-INsensitive, as is class instantiation.
php > var_dump(class_exists("DomNode"));
bool(true)
php > var_dump(class_exists("DOMNode"));
bool(true)
php > var_dump(class_exists("DOMNodE"));
bool(true)
php > $x = new DOMNOdE();
php > var_dump(get_class($x));
string(7) "DOMNode"
(tested with PHP 5.5.10 on Linux)
This can cause some headaches in correlating class names to file names, especially on a case-sensitive file system.

Hi guys!
Be careful and don't forget about second boolean argument $autoload (TRUE by default) when check exists class after spl_autoload_register. Propose short example
file second.php
<?php
class Second {}
?>
file index.php
<?php
class First
{
  function first($class, $bool) {
    spl_autoload_register( function($class) {
      require strtolower($class) . '.php';
    });
    echo class_exists($class, $bool)?'Exist!!!!':'Not exist!';
  }
}
new First($class = 'Second', $bool = true); //Exist!!!!
new First($class = 'Second', $bool = false); //Not exist!
?>
Because __autoload executing much earlier than boolean returned, imho..

I'm running PHP 5.3.4 on Windows 7 and had some difficulty autoloading classes using class_exists(). In my case, when I checked for the class and it didn't exist, class_exists automatically threw a system Exception. I was also throwing my own exception resulting in an uncaught exception.
<?php
/**
 * Set my include path here
 */
$include_path = array( '/include/this/dir', '/include/this/one/too' );
set_include_path( $include_path );
spl_autoload_register();
/**
 * Assuming I have my own custom exception handler (MyException) let's
 * try to see if a file exists.
 */
try {
  if( ! file_exists( 'myfile.php' ) ) {
    throw new MyException('Doh!');
  }
  include( 'myfile.php' );
}
catch( MyException $e ) {
  echo $e->getMessage();
}
/**
 * The above code either includes myfile.php or throws the new MyException
 * as expected. No problem right? The same should be true of class_exists(), 
 * right? So then...
 */
$classname = 'NonExistentClass';
try {
  if( ! class_exists( $classname ) ) {
    throw new MyException('Double Doh!');
  }
  $var = new $classname();
}
catch( MyException $e ) {
  echo $e->getMessage();
}
/**
* Should throw a new instance of MyException. But instead I get an
* uncaught LogicException blah blah blah for the default Exception
* class AND MyException. I only catch MyException so we've got on
* uncaught resulting in the dreaded LogicException error.
*/
?>
By registering an additional autoload handler function that did nothing, I was able to stop throwing the extra Exception and only throw my own.
<?php
/**
 * Set my include path here
 */
$include_path = array( '/include/this/dir', '/include/this/one/too' );
set_include_path( $include_path );
spl_autoload_register();
spl_autoload_register( 'myAutoLoad' ); // Add these two and no worries...
function myAutoLoad() {}
/**
 * By registering the additional custom autoload function that does nothing
 * class_exists() returns only boolean and does NOT throw an uncaught Exception
 */
?>
Found this buried in some search results. I don't remember the page URL but if it would have been here it might have saved me some time!

[ >= PHP 5.3]
If you are checking if a class exists that is in a specific namespace then you have to pass in the full path to the class:
echo (class_exists("com::richardsumilang::common::MyClass")) ? "Yes" : "No";

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

If spl_autoload_register() had been called, then function will try autoload class if it does not exists.
Use instead
<?php
in_array($class_name, get_declared_classes());
?>