how to extract a link from a string containg two links

My string is like this

$str="http://www.abc.com/tt.php?aa=http://www.xyz.com/abc.doc"

I want to extract the second url out of it

if the string is a normal link then the code should not change it,only if it has two http it shd extarct the second url of it

Regards

Alex

 

<?php
$a = "http://test.com/a.php?x=http://google.com";
$a = substr($a,strrpos($a,"http://"));

?>

 

http://php.net/parse_url