Including php file instead of load()

[DavidBonting]

Hello,
I'm using this script right now:
mainpage.php

<?php
$mainpage = 'secret';
?>
<a href="#" id="renamegroups">Rename groups</a>
<div id='renamegroupsl'>
</div>


<script type="text/javascript">
$('#renamegroups').click(function(){
   $("#renamegroupsl").load("../information/renamegroups.php");
   return false;
});
</script>

renamegroups.php

<?php
echo $mainpage;
?>

The problem is that I get this error:
Notice: Undefined variable: mainpage in (...)
So how could I include a file with ajax / jquery (because if I include the file this will work and the error won't show up)

Thanks!