Parse Forum username to an external page

[Shepski]

I am going to be running competitions on my site but I only want people with a minimum amount of posts to be able to enter. For this I am thinking about parsing the logged in username from the forum to the external php page, then doing a db lookup on the name to check they have enuf posts. What is the best way of achieving this without giving the user the ability to change the username whilst its parsing accross?

thanks