Probá así:
Código PHP:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title></title>
<script>
function $(id){
return document.getElementById(id);
}
var asignar=setInterval(function(){
if($('pp')){
if(window.ActiveXObject){
$('pp').onreadystatechange=function(){
if($('pp').readyState=='complete'){
alert('cargó');
}
}
clearInterval(asignar);
return;
}
$('pp').onload=function(){
alert('cargó');
}
clearInterval(asignar);
}
},10);
</script>
</head>
<body>
<iframe id="pp" width="300" height="200" src="b.php"></iframe>
</body>
</html>