PHP支付宝退款接口问题

我做支付宝付款可以正常支付 签名正常 为什么退款就有问题 提示签名错误 $aaa = 'timestamp='.$data.'&app_id=2016090444236148&charset=utf-8&method=alipay.trade.refund&sign_type=RSA&version=1.0&biz_content= {"out_trade_no":"HKJ1476255118","trade_no":"2016101221001004690214300282","refund_amount":1,"refund_reason":"正常退款","out_request_no":"HZ01RF00881"}';
  
   $big = urlencode('{"out_trade_no":"HKJ1476255118","trade_no":"2016101221001004690214300282","refund_amount":1,"refund_reason":"正常退款","out_request_no":"HZ01RF00881"}');
  $ste = 'timestamp='.$data.'&app_id=2016090444236148&charset=utf-8&method=alipay.trade.refund&sign_type=RSA&version=1.0&biz_content='.$big;  $oo = $this ->rsaSigng($aaa);怎么回事  错误代码 {"alipay_trade_refund_response":{"code":"40002","msg":"Invalid Arguments","sub_code":"isv.invalid-signature","sub_msg":"无效签名"},"sign":"aUududgICSy+Wr5NppUHX3FedZeQJCZ+L+xYy7q/TwwXLT5PEkF1PGlbG8hohV0kLZO9iFkWt7mhJeozz5GAp9cqVCnvEdiOT1ausq1EdBXIWF4kPb8C2ssoq6OpgBoHgd+cPbpaekbwbyHxWIHvfZopDUCG+G8aTUxJjLrTVsY="}
  
  
   $order=$ste.'&sign='.urlencode($oo);