I checked a lot of posts on the Internet, and finally got it done, so as to provide dinner for the newcomers ~
PS.. If you confirm that all your parameters are correct, but still prompt for a signature error, update the API key, and then ok. Wechat seems to have a cache

create a sandbox acceptance class that extends from WxPayDataBase

class Sandbox extends WxPayDataBase{

    //
    public function s_setValues($k, $v){
        $this->values[$k] = $v;
    }

    //
    public function s_getValues($k){
        return $this->values[$k];
    }
    
    //API
    public static function getSignKey($input, $mch_key){

        console_log( '1:'.json_encode($input->values) );
        
        //
        $url = 'https://api.mch.weixin.qq.com/sandboxnew/pay/getsignkey';
        
        //
        $input->setSign($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 );

        //
        $result = self::postXmlCurl($xml, $url);
        console_log( json_encode($result) );

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

        return $result;
    }

    

    /**
     * 
     * 32
     * @param int $length
     * @return 
     */
    public static function getNonceStr($length = 32) 
    {
        $chars = "abcdefghijklmnopqrstuvwxyz0123456789";  
        $str ="";
        for ( $i = 0; $i < $length; $iPP )  {  
            $str .= substr($chars, mt_rand(0, strlen($chars)-1), 1);  
        } 
        return $str;
    }


    /**
     * postxmlurl
     * 
     * @param string $xml  postxml
     * @param string $url  url
     * @param bool $useCert 
     * @param int $second   url30s
     * @throws WxPayException
     */
    private static function postXmlCurl($xml, $url, $useCert = false, $second = 30)
    {    
        $ch = curl_init();
        
        //
        curl_setopt($ch, CURLOPT_TIMEOUT, $second);

        //
        curl_setopt($ch, CURLOPT_URL, $url);
        
        //skysowe_modifid
        curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, FALSE);
        curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, FALSE);
        
        //header
        curl_setopt($ch, CURLOPT_HEADER, FALSE);
        
        //
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE);

        console_log( 'cert:'.$useCert );
        if($useCert == true){
            //
            //:cert  key .pem
            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
        curl_setopt($ch, CURLOPT_POST, TRUE);
        curl_setopt($ch, CURLOPT_POSTFIELDS, $xml);
        //console_log( 'xml:'.json_encode($xml) );

        //curl
        $data = curl_exec($ch);
        //console_log( 'curl_result:'.json_encode($data) );
        
        //
        if($data){
            curl_close($ch);
            return $data;
        } else { 
            $error = curl_errno($ch);
            curl_close($ch);
            throw new WxPayException("curl:$error");
            //console_log( 'curl_error:'.$error );
        }
    }


    /**
     * xml
     * @throws WxPayException
    **/
    public function array2xml()
    {
        if(!is_array($this->values) 
            || count($this->values) <= 0)
        {
            throw new WxPayException("");
        }
        
        $xml = "<xml>";
        foreach ($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; 
    }

    /**
     * xmlarray
     * @param string $xml
     * @throws WxPayException
     */
    public function xml2array($xml)
    {    
        if(!$xml){
            throw new WxPayException("xml");
        }
        //XMLarray
        //xml
        libxml_disable_entity_loader(true);
        $this->values = json_decode(json_encode(simplexml_load_string($xml, 'SimpleXMLElement', LIBXML_NOCDATA)), true);        
        return $this->values;
    }

}

then get sandbox_signkey

    //:0 
    $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;