<div style="width:550px; margin:0 auto;>
You're missing the closing quotation mark on the inline style.
Also, if you are sending a form back to itself using php, I would suggest these two changes to your coding habbits:
<form action="<?php echo $_SERVER['PHP_SELF']; ?>" method="post" >
No matter what page this form is on, it'll send back to itself. What happens when you move and/or rename this file, and forget to change the form?
don't rely on the php short tags <?= ?> as some servers might have those turned off by default, and then you code won't process, and you could end up exposing your code to the unwanted eye. Use the full php tags. <?php ?> it's not that much more effort to type the two additional character.
For debugging purposes, I like to use this at the top (or bottom) of my pages, when testing form or session transfers.
<?php print_r($_POST); ?>
This allows you to see exactly what is getting passed through, so it's easier to see what and where things are failing.