gistfile1.php

<?php
$mf2 = new \Mf2\Parser($html, $url);
$rels = $mf2->parseRelsAndAlternates();
if(array_key_exists('webmention', $rels[0])) {
	$endpoint = $rels[0]['webmention'][0];
} elseif(array_key_exists('http://webmention.org/', $rels[0])) {
	$endpoint = $rels[0]['http://webmention.org/'][0];
}

if($endpoint) {
	if(filter_var($endpoint, FILTER_VALIDATE_URL)) {
		return $endpoint;
	} else {
		//it must be a relative url, so resolve with php-mf2
		$resolved = $mf2->resolveUrl($endpoint);
		return $resolved;
	}
} else {
	return false;
}