the permissions on your php file do not need to be no more than 0644 and to have cron execute a php script you need to tell the system what type of file its gonna be executing. In your case this is what you would enter into the command box for cron in webmin:
php /home/admin/mydomain.com/htdocs/followup.php
then set your times to run the cronjob in your selection boxes below it and save it. Also dont forget at the top of your cronjob setup page in webmin is the box that you tell cron what user is executing it,It must be a user with access to run cron jobs.
Hope that helps