網(wǎng)上查了好多帖子，終于搞定了，以饗后來者~
PS.如果確認自己各項參數(shù)都對，但是還是提示簽名錯誤，更新一下API密鑰，然后就ok了，微信那邊好像有緩存

創(chuàng)建一個沙箱驗收的類，擴展自WxPayDataBase

class Sandbox extends WxPayDataBase{

    //設置對象的屬性
    public function s_setValues($k, $v){
        $this->values[$k] = $v;
    }

    //讀取對象的屬性
    public function s_getValues($k){
        return $this->values[$k];
    }
    
    //獲取密鑰API
    public static function getSignKey($input, $mch_key){

        console_log( '1:'.json_encode($input->values) );
        
        //提交業(yè)務
        $url = 'https://api.mch.weixin.qq.com/sandboxnew/pay/getsignkey';
        
        //生成簽名
        $input->setSign($mch_key);
        console_log( '2:'.json_encode($input->values) );
        
        //$values = $input->GetValues();
        //console_log( '3:'.json_encode($values) );
        
        $xml = $input->array2xml();
        console_log( '4:'.$xml );

        //向微信請求接口
        $result = self::postXmlCurl($xml, $url);
        console_log( json_encode($result) );

        $result = $input->xml2array($result);
        console_log( json_encode($result) );

        return $result;
    }

    

    /**
     * 
     * 產(chǎn)生隨機字符串，不長于32位
     * @param int $length
     * @return 產(chǎn)生的隨機字符串
     */
    public static function getNonceStr($length = 32) 
    {
        $chars = "abcdefghijklmnopqrstuvwxyz0123456789";  
        $str ="";
        for ( $i = 0; $i < $length; $i++ )  {  
            $str .= substr($chars, mt_rand(0, strlen($chars)-1), 1);  
        } 
        return $str;
    }


    /**
     * 以post方式提交xml到對應的接口url
     * 
     * @param string $xml  需要post的xml數(shù)據(jù)
     * @param string $url  url
     * @param bool $useCert 是否需要證書，默認不需要
     * @param int $second   url執(zhí)行超時時間，默認30s
     * @throws WxPayException
     */
    private static function postXmlCurl($xml, $url, $useCert = false, $second = 30)
    {    
        $ch = curl_init();
        
        //設置超時
        curl_setopt($ch, CURLOPT_TIMEOUT, $second);

        //設置訪問的網(wǎng)址
        curl_setopt($ch, CURLOPT_URL, $url);
        
        //skysowe_modifid
        curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, FALSE);
        curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, FALSE);
        
        //設置header
        curl_setopt($ch, CURLOPT_HEADER, FALSE);
        
        //要求結果為字符串且輸出到屏幕上
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE);

        console_log( 'cert:'.$useCert );
        if($useCert == true){
            //設置證書
            //使用證書：cert 與 key 分別屬于兩個.pem文件
            curl_setopt($ch,CURLOPT_SSLCERTTYPE,'PEM');
            curl_setopt($ch,CURLOPT_SSLCERT, WxPayConfig::SSLCERT_PATH);
            curl_setopt($ch,CURLOPT_SSLKEYTYPE,'PEM');
            curl_setopt($ch,CURLOPT_SSLKEY, WxPayConfig::SSLKEY_PATH);
        }
        
        //post提交方式
        curl_setopt($ch, CURLOPT_POST, TRUE);
        curl_setopt($ch, CURLOPT_POSTFIELDS, $xml);
        //console_log( 'xml:'.json_encode($xml) );

        //運行curl
        $data = curl_exec($ch);
        //console_log( 'curl_result:'.json_encode($data) );
        
        //返回結果
        if($data){
            curl_close($ch);
            return $data;
        } else { 
            $error = curl_errno($ch);
            curl_close($ch);
            throw new WxPayException("curl出錯，錯誤碼:$error");
            //console_log( 'curl_error:'.$error );
        }
    }


    /**
     * 輸出xml字符
     * @throws WxPayException
    **/
    public function array2xml()
    {
        if(!is_array($this->values) 
            || count($this->values) <= 0)
        {
            throw new WxPayException("數(shù)組數(shù)據(jù)異常！");
        }
        
        $xml = "<xml>";
        foreach ($this->values as $key=>$val)
        {
            if (is_numeric($val) || $key=="nonce_str" || $key=="sign" ){
                $xml.="<".$key.">".$val."</".$key.">";
            }else{
                $xml.="<".$key."><![CDATA[".$val."]]></".$key.">";
            }
        }
        $xml.="</xml>";
        return $xml; 
    }

    /**
     * 將xml轉為array
     * @param string $xml
     * @throws WxPayException
     */
    public function xml2array($xml)
    {    
        if(!$xml){
            throw new WxPayException("xml數(shù)據(jù)異常！");
        }
        //將XML轉為array
        //禁止引用外部xml實體
        libxml_disable_entity_loader(true);
        $this->values = json_decode(json_encode(simplexml_load_string($xml, 'SimpleXMLElement', LIBXML_NOCDATA)), true);        
        return $this->values;
    }

}

然后獲取sandbox_signkey

    //沙箱用例：0 獲取驗簽秘鑰
    $sandbox_test = new Sandbox();

    $sandbox_test->s_setValues('mch_id', WxPayConfig::MCHID);
    $sandbox_test->s_setValues('nonce_str', $sandbox_test->getNonceStr());

    //debug
    console_log( $sandbox_test->s_getValues('mch_id') );
    console_log( $sandbox_test->s_getValues('nonce_str') );
    console_log( WxPayConfig::KEY );

    $rs = Sandbox::getSignKey($sandbox_test, WxPayConfig::KEY);

    //debug
    echo json_encode($rs);
    echo $rs['sandbox_signkey'];

    exit;