function getContent($source){
   if(empty($source)) return "";
   $str = "这是我要返回的值了";
   preg_match('/<span>(\d+)<\/span>/isU',$source,$matches);
   if($matches[1]>time()){
      return $str;
   }else{
      return "";
   }
}

//使用方法
date_default_timezone_set("Asia/shanghai");
$content ='<span>1324570103</span>';
$res = getContent($content);