What about this (adjust path to php executable below)?

30 * * * * /usr/bin/php /home/www/inc/crontab.php

Note that your host may not necessarily have the php executable installed if it is just meant to run as a module to Apache. Only you (or your host admin) will know how the PHP is configured. In that case, you will not be able to run PHP scripts on the command line.

By the way the following works for me:

a.php:

#!/usr/bin/php
<?php phpinfo() ?>

chmod +x a.php
./a.php

So you probably may do it too.