That piece of code you'd place in the form, within a section which check wether the form has been submitted. If submitted the code mentioned will re-locate the page to the new page (1.php for instance).
the form you will submit to the original page.
<<Do you agree with me, HAND, or did you have something completely different in mind?>>
OR
You could of course take all the source code of all pages, paste it in one big file (It is server-side, so will not be pushed up and down to the client) check for the 1, 2, 3, ... value selected, and then execute a specific part of the page.
if ($page = 1)
{
Do code which was in 1.php
}
else
{
if ($page = 2)
{
Do code which was in 2.php
}
}