Given an input URL, find the canonical URL after following redirects and looking at rel=canonical

canonical.php

<?php
if(!isset($_GET['url'])) {
  ?>
  <form action="" method="get">
    <input type="url" name="url">
    <input type="submit" value="Go">
  </form>
  <?
  die();
}

header('Content-type: text/plain');


// Given an input URL, find the canonical URL, after following redirects and looking for rel=canonical in the source HTML
$url = $_GET['url'];

$ch = curl_init($url);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, true);
curl_setopt($ch, CURLOPT_MAXREDIRS, 10);
// Some sites don't like crawlers, so pretend to be a browser
curl_setopt($ch, CURLOPT_HTTPHEADER, [
  'Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8',
  'User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/55.0.2883.95 Safari/537.36'
]);
$body = curl_exec($ch);
$final_url = curl_getinfo($ch, CURLINFO_EFFECTIVE_URL);
if($final_url)
  $url = $final_url;

// Check for rel=canonical
if($body) {
  $dom = load_html($body);
  if($dom) {
    $links = $dom->getElementsByTagName('link');
    foreach($links as $link) {
      $rels = [];
      if($link->hasAttribute('rel') && ($relAtt = $link->getAttribute('rel')) !== '') {
        $rels = preg_split('/\s+/', trim($relAtt));
      }
      if(in_array('canonical', $rels)) {
        $url = $link->getAttribute('href');
      }
    }
  }
}

echo $url."\n";



function load_html($html) {
  $dom = new DOMDocument;
  libxml_use_internal_errors(true); // suppress parse errors and warnings
  // Force interpreting this as UTF-8
  @$dom->loadHTML('<?xml encoding="UTF-8">' . $html, LIBXML_NOWARNING|LIBXML_NOERROR);
  libxml_clear_errors();
  return $dom;
}