PHP: how to get a list of classes that implement certain interface?

Generic solution:

function getImplementingClasses( $interfaceName ) {
    return array_filter(
        get_declared_classes(),
        function( $className ) use ( $interfaceName ) {
            return in_array( $interfaceName, class_implements( $className ) );
        }
    );
}

You can use PHP's ReflectionClass::implementsInterface and get_declared_classes functions to accomplish this:

$classes = get_declared_classes();
$implementsIModule = array();
foreach($classes as $klass) {
   $reflect = new ReflectionClass($klass);
   if($reflect->implementsInterface('IModule')) 
      $implementsIModule[] = $klass;
}

You dont need Reflection for this. You can simply use

• class_implements — Return the interfaces which are implemented by the given class

Usage

in_array('InterfaceName', class_implements('className'));

Example 1 - Echo all classes implementing the Iterator Interface

foreach (get_declared_classes() as $className) {
    if (in_array('Iterator', class_implements($className))) {
        echo $className, PHP_EOL;
    }
}

Example 2 - Return array of all classes implementing the Iterator Interface

print_r(
    array_filter(
        get_declared_classes(), 
        function ($className) {
            return in_array('Iterator', class_implements($className));
        }
    )
);

The second example requires PHP5.3 due to the callback being an anonymous function.

To check who implements a particular interface, you can write a function like below:

<?php
/**
 * Get classes which implement a given interface 
 * @param string $interface_name Name of the interface
 * @return array Array of names of classes. Empty array means input is a valid interface which no class is implementing. NULL means input is not even a valid interface name.
 */
function whoImplements($interface_name) {
    if (interface_exists($interface_name)) {
        return array_filter(get_declared_classes(), create_function('$className', "return in_array(\"$interface_name\", class_implements(\"\$className\"));"));
    }
    else {
        return null;
    }
}

Output of an example call var_export(whoImplements('ArrayAccess')); will be as follows:

[sandbox]$ php whoimplementswhat.php
Array
(
    [29] => CachingIterator
    [30] => RecursiveCachingIterator
    [38] => ArrayObject
    [39] => ArrayIterator
    [40] => RecursiveArrayIterator
    [48] => SplDoublyLinkedList
    [49] => SplQueue
    [50] => SplStack
    [55] => SplFixedArray
    [56] => SplObjectStorage
    [111] => Phar
    [112] => PharData
)

This way, you don't use loops and you can run your code on lower versions of PHP. Function array_filter loops internally, but inside PHP execution engine (hence more performant than loops written in PHP code).