preg_replace

<?php
      // if the $field_url is not present, proceed as normal
      if (empty($field_url)) {
        echo $content;
      // if the $field_url is present, format it as an actual url
      // the problem here is that $content may contain multiple urls, I want to do a regex
      // on only the $detected_url, even if the $detected_url is found multiple times in $content
      } else {
        $detected_field_url = $field_url;
        $formatted_url = "<a href=\"$detected_field_url\">$detected_field_url</a>";
        // this actually echoes nothing, i think because of unescaped characters?
        echo preg_replace("/$detected_field_url/", "/$formatted_url/", $content);
      }
?>