But if PHP is not running as a CGI, then the #! path won't exist.
In that case, what you might be able to do (admin staff willing) is to have your cron job start lynx (or similar command-line driven, text-based http client) and pass it the URL to the script you want run
(Forget the precise formulation: a search on this forum for "lynx" would be sure to turn it up though.)