There's no real way to detect a popup blocker in PHP but it is pretty easy in JavaScript.
<script type="text/JavaScript" language="JavaScript"> var mine = window.open('','','width=1,height=1,left=0,top=0,scrollbars=no'); if(mine) var popUpsBlocked = false else var popUpsBlocked = true mine.close()</script>
Just insert that in the HEAD of your code and wether or not their browser blocks popups will be stored in the popUpsBlocked variable.