I found this page while searching for an answer to a similar problem I was having. I know this thread is like 7 months old, but I feel it important to post a small nugget of wisdom to help anyone else who comes across this page looking for help.
after you have declared your variables, example:
<?php
$val1 = $_GET['display'];
$val2 = $_GET['menu'];
?>
to change these in a ahref link you can do this:
<a href="?display=<?php echo $val1; ?>&menu=test2.html">Menu to Test 2</a>
<a href="?display=test.html&menu=<?php echo $val2; ?>">Display toTest 1</a>
So inserting an echo into the link works just fine, allowing you to change one php value and allowing another to stay the same on the page. I hope this helps someone out there