This isn't a php issue. But here's a javascript function that should do the trick. It even gives focus back to the window if it is still open and another link is clicked.
function OpenPage(URL)
{
var newWindow = null;
if (!newWindow || newWindow.closed) {
newWindow = window.open(URL,"WINDOWNAME","toolbar=no,location=no,directories=no,status=no,menubar=no,scrollbars=yes,resizable=yes,copyhistory=no,width=300,height=300");
newWindow.focus();
} else {
newWindow.location(URL);
newWindow.focus();
}
}