Function execute when user exits site

[mrgrammar]

I don't know if this can be done via PHP or not, but here goes...

I have a site that shows a single pop-under when the user enters the site. As long as they are on the site (or 1 hour whichever comes first), they do not see another pop-under.

if(isset($_COOKIE['x'])){
	$showpop = $_COOKIE['x'];
	$cname = "tfnpop";
	$cvalue = "1";
	$cexpire = time()+60*60;
	setcookie($cname, $cvalue, $cexpire);
}else{
	$cname = "x";
	$cvalue = "1";
	$cexpire = time()+60*60;
	setcookie($cname, $cvalue, $cexpire);
	$showpop = 0;
}
if ($showpop == 0){
	include "popurl.php";
}

I want to show them a single pop-under when they exit the site. Is there a way to trigger it when they leave the site?

[execute]

You need javascript, this is a php forum... refer to forums like in my sig for that.