passing a variable.

Hey there, I'm sure how to go about doing something. I want users to come to a page where they enter their phone extension. Once they enter their extension, and hit submit, it should then bring them to a second page. What happens on this second page is this: It will create a form for them to fill out. But certain information will automatically be filled out like first name, last name, dept, and title. Because it looks up to a employee table to find that information out. The problem I'm having is that I don't know how to get the extension that was entered in the first page to appear in the url for the second page once you hit submit (on the first page). the url should look something like http://monster/histinfo.php?extension=1114