Start by typing at a command line:
Do this as the user that the cron job will be run under. This will show you the path to your executable. You can then use that path (if it's not already in your PATH variable) in your cron entry:
5 * * * * /your/path/to/php /var/www/some/path/script.php
Edit: you may need to install the
php5-cli (Ubuntu package name) package if all you have is the Apache PHP module installed. This will give you the binary executable that you can run from a command line.