Form Based Authentication

I am new to PHP and I am attempting to use some type of authentication in my site. My first page is a simple HTML form page with 2 fields username and passwrd. that form is POST'ed to a 2nd page. In the second page I start a session, and if $_POST["username"] is set I Register a "authenticated_user" session variable. (This will be replaced by an actual authenticate function). Could Someone Explain why I have to submit my first form twice in order to be considered authenticated. Also any pointers to tutorials or samples of form based authentication would be appreciated. My Code is attached:

Attachment: [unknown/unknown] verify.php