Description:
------------
register_tick_function() does not check if the first parameter is a valid callback.

Reproduce code:
---------------
<?php
	register_tick_function('non_existing_function');
?>

Expected result:
----------------
Warning: 'non_existing_function' is not a valid callback in C:\webserver\webroot\program.php on line 2

... or something similar

Actual result:
--------------
No error issued