cron command to run every 12 hours

I need unix cron command to run every 12 hours.

I have 500+ sub blogs in my server.

This is the file i want to run every 12 hours

http://*.mysite.com/somedir/index.php

Where * is my subdomain of my blogs.

I need cron command for all blogs. Is it possible to run all of them with single command? OR do i have to create command for each blog?