Dr Byte's solution works well for me but I thought I'd add a little caveat which might save somebody some time.
If you run your php program directly as a cron job it won't have the right working directory and won't find /includes/config.php so it will fail with;
ERROR: admin/includes/configure.php file not found. Suggest running zc_install/index.php?
To avoid this you need to tell cron to change directory before invoking PHP to run the program, so your cron command should read
Code:
cd "public_html/admin"; /usr/local/bin/php my_program.php
Remember to leave a space between your path to PHP and the program name, you are invoking PHP with the program name as a parameter.
I hope this helps someone.