[PHP] Script to only allow from a page... (Process form)

Is there a script that only allows users to update their admin details ONLY on the page I’ve provided. (userdetails.php)

The way my users upload their personal details is through a process (process_update.php), PHP checks the user id and updates the mySQL table. So means that users can actually have another copy of the update form in their system and actioned to my process page and with a success!

What script do I use to prevent that? Thanks in advance.