including class files with unserialize_callback_func

i am using the following to try to pull in php classes from separate files based on their names using unserialize_callback_func. I cant seem to get it to work with cgi php 4.1.6 on apache/redhat . I notice that it isnt shown under variables available for ini_set() but it is shown for unserialize() from which Ive adapted the following script: ini_set('unserialize_callback_func','thecallback'); function thecallback($classname) { echo "calling back"; $filename="includez/".$classname.".cls.php"; include($filename); } I would think i'd atleast get an acknowledgement that the function indeed fired. Has this feature not yet been implemented?