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

Note: class_exists() check only classes! 
<?php
interface DemoInterface {};
var_dump(class_exists('DemoInterface')); // false
trait DemoTrait {};
var_dump(class_exists('DemoTrait')); // false
class DemoClass {};
var_dump(class_exists('DemoClass')); // true
?>

Common function:
<?php
/**
  * Checks if the class/trait/interface has been defined.
  *
  * @param string $name The case-insensitive name of class/trait/interface
  * @param bool $autoload Whether to call spl_autoload()
  * @return bool
  */
function structure_exists(string $name, bool $autoload = true): bool
{
       return class_exists($name, $autoload)
              || interface_exists($name, $autoload)
              || trait_exists($name, $autoload);
}
?>

Beware that `\class_exists()` returns `true` for enums.

<?php
enum Test: int
{
    case One = 1;
    case Two = 2;
}

\var_dump(\class_exists(Test::class)); // bool(true)
?>

Having this in mind, the correct check for a class existence is:

<?php
function is_class_exist(string $class): bool
{
    return \class_exists($class) && !\enum_exists($class);
}
?>

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.

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.

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..

[ >= 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

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!

Beware that class_exists is a runtime check and not a compile time check!

This means that even if the class is defined after the class_exists, it will return true!

<?php

echo 'Does class "MyClass" exist? : ', class_exists('MyClass') ? 'YES' : 'NO';

class MyClass
{
}

?>

Output: YES