All possible hook syntaxes in YOURLS

test.php

<?php
// Load YOURLS
require_once( __DIR__.'/includes/load-yourls.php' );
?>
<pre>
<?php

// Simple hook with string <function name>
function my_callback_function($in) {
    return "$in 1 ";
}
yourls_add_filter('my_hook_test', 'my_callback_function');

// Hook with array(<class name>, <method>)
class A {
    static function my_callback_function($in) {
        return "$in 2 ";
    }
}
yourls_add_filter('my_hook_test', array('A','my_callback_function'));

// Hook with string "<class name>::<method>"
class B {
    static function my_callback_function($in) {
        return "$in 3 ";
    }
}
yourls_add_filter('my_hook_test', 'B::my_callback_function');

// Hook with array(<class instance>, <method>)
class C {
    static function my_callback_function($in) {
        return "$in 4 ";
    }
}
$obj = new C();
yourls_add_filter('my_hook_test', array($obj, 'my_callback_function'));

// Hooks with array(<class name>, <method>) with class inheritance
class D {
    public static function my_callback_function($in) {
        return "$in 5 ";
    }
}
class E extends D {
    public static function my_callback_function($in) {
        return "$in 6 ";
    }
}
yourls_add_filter('my_hook_test', array('E', 'parent::my_callback_function'));
yourls_add_filter('my_hook_test', array('E', 'my_callback_function'));

// Hook with object implementing __invoke()
class F {
    public function __invoke($in) {
        return "$in 7 ";
    }
}
$f = new F();
yourls_add_filter('my_hook_test', $f);

// Hook with closure
$my_callback_function = function($in) {
    return "$in 8 ";
};
yourls_add_filter('my_hook_test', $my_callback_function);

// Now apply filter
var_dump(yourls_apply_filter('my_hook_test', 'hello'));
// Result : string 'hello 1  2  3  4  5  6  7  8 ' (length=29)