function ax_cover($tid){
$r = db_find_one('post', array('tid'=>$tid));
$content = $r['message_fmt']; //文章内容
preg_match_all("/<img.*src=[\'|\"](.*)[\'|\"]\s*.*>/iU", $content, $img);
$imgArr = $img[1];
if(isset($imgArr[0])){
return $imgArr[0];
}else{
return "";
}