Keep all content to one section of a multi-row layout

I have a web application whose loading page presents a form with radioboxes for 3 types of input : Color, Size, Material. This loading page is basically a php file first.php that has 3 sections, a Header div, a Content div which presents the form and a footer div.

The action target for the Submit button is a 2nd file process.php which offers a page of choices. I want this file to exactly take up the space occupied by first.php - which is to say, I want the header and footer to stay as they are throughout the life of the session. I could load the header and footer for every target php file but I'd rather save on the amount of content from server to browser.

What should I do to ensure that the 2nd(and every subsequent nth page) stays only in the content sub-section? The code for first.php is like this: header comes here

<div id="content">
<form action="process.php" method="post">
.....
</form>
</div>

<div id="footer">
footer comes here
</div>

Thanks.