Auto-link URLs in an HTML document that haven't already been linked.

gistfile1.php

<?php

/**
 * Safely auto-link HTML content.
 * Doesn't double-link existing content. 
 * Plus logging features
 * @param string $html
 * @return string
 */
function safe_autolink($html) {
  $doc = new DOMDocument();
  $success = $doc->loadHTML($html);
  if ($success) {
    $xpath = new DOMXpath($doc);
    foreach ($xpath->query('//text()') as $text) {
      if (!($text->parentNode->tagName == "a")) {
        $frag = $doc->createDocumentFragment();
        $success = !!$frag->appendXML(preg_replace('#((?:http|https)://\S+)#', '<a href="$1">$1</a>', htmlentities($text->data)));
        if ($success) {
          $text->parentNode->replaceChild($frag, $text);
        } else {
          break;
        }
      }
    }
  }
  if ($success) {
    if (version_compare(PHP_VERSION, '5.3.6') >= 0) {
      $func_name = "saveHTML";
    } else {
      $func_name = "saveXML";
    }
    $autolink_html = $doc->$func_name($xpath->query("//*")->item(0));
    $return = $autolink_html;
  } else {
    $message = "Unable to auto link HTML.";
    $exception = new Exception($message);
    error_log(sprintf("%s, %s", $message, $exception->getTraceAsString()));
    $return = $html;
  }
  return $return;
}