How can I accomplish this? pls

[eluser]my_immortal[/eluser]
I had developed my site.
assume that i had default controller to 'main_page.php'
if someone calls to "www.mysite.com" the "main_page.php" will be called.
but if i want to someone call to his/her profile like "www.mysite.com/username1"
with the controller name "profile.php"
I try to remap URI rounting but i can't find any solution for this one

How can I accomplish this? Pls suggest Me.
Since I remove the "index.php" with .htaccess.

If any mistask with my english pls advice me. I don't good at english.