why dont you use a javascript instead?
<script language="JavaScript" type="text/javascript">
<!-- hide from JavaScript-challenged browsers
function openWindow1(url) {
popupWin = window.open(url,'new_page','width=750,height=500,scrollbars=yes,toolbars=yes,menubar=yes,resizable=yes')
}
// done hiding -->
</script>
put that in the head and then creata a link like this:
<a href="JavaScript: openWindow1('ViewPM.php?DisplayWhat=variable')">click me</a>
its what i use for my site, edit it how you please.
You need to remove the space between javascript: and open (the forum creates a smilie otherwise!)