addCanonicalTag.php

<?php

function addCanonicalTag($html, $url) {
  $dom = new DOMDocument();
  @$dom->loadHTML($html, LIBXML_HTML_NODEFDTD);

  $head = $dom->getElementsByTagName('head')->item(0);

  if ($head) {
    $canonical = $dom->getElementsByTagName('link')->item(0);
    if ($canonical) {
      $canonical->setAttribute('href', $url);
    } else {
      $newCanonical = $dom->createElement('link');
      $newCanonical->setAttribute('rel', 'canonical');
      $newCanonical->setAttribute('href', $url);
      $head->appendChild($newCanonical);
    }
  }

  return $dom->saveHTML();
}