Header Problems and Basic PHP Question

Question
Hey,
I've been working on a site that I've been planning for awhile. It's a pretty big one, and I'm learning pretty much everything along the way. I'm having a few problems, so I figured I'd ask a couple of question here.
The first thing I'm having trouble with is I can't seem to figure out why this section of php is firing the header, and sending me back to the index page even though I've typed something in all the fields.

PHP:

<?php

session_start();

if(!IsSet($_POST[‘userName’])…

Header Problems and Basic PHP Question

See original post by Jeremy Benson