you use a get variable, that part that says
?showtopic=25623
so you have something like
CODE
<script language="javascript">
function sendtoPHP(thisvar) {
window.location = "yoursite.com/thatphpfile.php?write=" + thisvar;
}
</script>
and then the PHP
CODE
<?php
$Write = $_GET['write'];
// then whatever you were going to put.
?>