public function serve()
{
    $echoStr = $_GET["echostr"];
    if($this->checkSignature()){
        echo $echoStr;
        exit;
    }
}
//检查签名
private function checkSignature()
{
    $signature = $_GET["signature"];
    $timestamp = $_GET["timestamp"];
    $nonce = $_GET["nonce"];
    $token = "xxxxxxxxxxx";
    $tmpArr = array($token, $timestamp, $nonce);
    sort($tmpArr, SORT_STRING);
    $tmpStr = implode($tmpArr);
    $tmpStr = sha1($tmpStr);
    if($tmpStr == $signature){
        return true;

    }else{
        return false;
    }
}

微信公众号服务器配置（校验）