Interesting problem. I've fiddled around with this for a while; this method inevitably isn't perfect, but it may start you down the right path.
My solution begins with the two source files in this repository: https://github.com/usrflo/registered-domain-libs/tree/master/PHP
First, you may need to modify regDomain.inc.php to change an instance of $signingDomainParts = split('\.', $signingDomain);
to $signingDomainParts = preg_split('/\./', $signingDomain);
if split is deprecated in your php version.
Once you've got those saved, try this testing code, I put all of the URLs mentioned in the thread here as test cases:
<?php
require_once("effectiveTLDs.inc.php");
require_once("regDomain.inc.php");
$tests = Array("/images/sample.jpg","//main.com/images/sample.jpg","images/sample.jpg", "sub.main.com/images/sample.jpg", "http://www.example.com/www.google.com/sample.jpg", "amazon.co.uk/images/sample.jpg", "amazon.com/images/sample.jpg", "http://sub2.sub.main.co.uk/images/sample.jpg", "sub2.sub.main.co.uk/images/sample.jpg");
foreach($tests as $test)
{
echo "Attempting $test.<BR/>";
$one = parse_url($test);
if(!array_key_exists("host", $one))
{
echo "Converting to: http://$test";
echo "<BR/>";
$one = parse_url("http://$test");
}
if(!$one){echo "<BR/>";continue;}
echo "parse_url parts: ";
print_r($one);
echo "<BR/>";
if($one && array_key_exists("host", $one))
{
$domain = getRegisteredDomain($one["host"], $tldTree);
if(sizeof($domain))
{
$two = explode(".", $domain);
echo "domain parts: ";
print_r($two);
echo "<BR/>";
if(sizeof($two))
{
$three = array_diff(explode(".", $one["host"]), $two);
if(sizeof($three))
{
echo "Hark! A subdomain!: ";
print_r($three);
echo "<BR/>";
}
}
}
}
echo "<BR/>";
}
?>
This code identifies the following of the test-cases as having subdomains:
Attempting sub.main.com/images/sample.jpg.
Hark! A subdomain!: Array ( [0] => sub )
Attempting http://www.example.com/www.google.com/sample.jpg.
Hark! A subdomain!: Array ( [0] => www )
Attempting http://sub2.sub.main.co.uk/images/sample.jpg.
Hark! A subdomain!: Array ( [0] => sub2 [1] => sub )
Attempting sub2.sub.main.co.uk/images/sample.jpg.
Hark! A subdomain!: Array ( [0] => sub2 [1] => sub )