仅显示谷歌AdSense广告推特访客

我只想在访客来自推特时显示我的谷歌Adsense广告。目前我正在使用这段代码,这对Facebook和其他网站有效,但对Twitter无效

    <?php
            $ref = $_SERVER['HTTP_REFERER'];
            if (strpos($ref, 'twitter.com') != false) {?>
 <script type="text/javascript"><!--
    google_ad_client = "xx-xx-xxxxxxxxxxxxxxxxxx";
    /* xxxxxxxx xxxxxx xxx xxx xxx xx xxxxxx */
    google_ad_slot = "xxxxxxxxxxxxxx";
    google_ad_width = xxx;
    google_ad_height = xxx;
    //-->
    </script>
    <script type="text/javascript" src="http://pagead2.googlesyndication.com/pagead/show_ads.js">
    </script>
</div>
</div>

<?php }else{
  echo "";  }
  ?>