js-identifiers/is_valid_callback.function.php

<?php

/**
 * Checks if the given subject is a valid js callback function.
 *
 * @author     Torleif Berger
 * @link       http://www.geekality.net/?p=1739
 * @license    http://creativecommons.org/licenses/by/3.0/
 * @return     TRUE if the subject is valid; otherwise FALSE.
 */
function is_valid_callback($subject)
{
	$identifier_syntax
	  = '/^[$_\p{L}][$_\p{L}\p{Mn}\p{Mc}\p{Nd}\p{Pc}\x{200C}\x{200D}]*+$/u';

	$reserved_words = array('break', 'do', 'instanceof', 'typeof', 'case',
	  'else', 'new', 'var', 'catch', 'finally', 'return', 'void', 'continue', 
	  'for', 'switch', 'while', 'debugger', 'function', 'this', 'with', 
	  'default', 'if', 'throw', 'delete', 'in', 'try', 'class', 'enum', 
	  'extends', 'super', 'const', 'export', 'import', 'implements', 'let', 
	  'private', 'public', 'yield', 'interface', 'package', 'protected', 
	  'static', 'null', 'true', 'false');

	return preg_match($identifier_syntax, $subject)
	    && ! in_array(mb_strtolower($subject, 'UTF-8'), $reserved_words);
}