You can't, at least not with PHP. PHP is server side and doesn't deal or talk with the browser itself. PHP receives it's input from the browser via the server. It sends output back to the server which it turn returns it to the browser. You'll need to use a client-side tech. like JavaScript.
<rant>
IMHO, automatically opening extra browser windows is bad, bad, naughty, bad!! Every web developer should be allowed to do it once in their career, just to see how stupid and annoying it is. After that one grace useage, a large man named Bubba (who never takes off his sunglasses) should appear at your door for "a little chat" everytime you do such a thing🙂
If I want a website to take control of my desktop reality, I'll ask it to. If it does it without asking, 9 times of 10 I'm going to leave immediately never to return. It's even more annoying as those stupid Flash intro's and YAUFS everyone was all hyped up on for a while. Thankfully, those seem to be decreasing.
</rant>
-geoff