我正在嘗試獲取 payu webcheckout 實現的簽名我認為錯誤出現在簽名字段中，我在控制器中定義了 api 密鑰<form method="post" action="https://sandbox.checkout.payulatam.com/ppp-web-gateway-payu/">  <input name="merchantId" type="text" value="508029">  <input name="accountId" type="text" value="512321">  <input name="description" type="text" value="{{ $plans->description }}">  <input name="referenceCode" type="text" value="{{ $plans->id }}">  <input name="amount" type="text" value="{{ $plans->price }}">  <input name="tax" type="text" value="0">  <input name="taxReturnBase" type="text" value="0">  <input name="currency" type="text" value="COP">  <input name="signature" type="text" value="md5($apy_key." ~ "."508029 "."~ ".$plans->id."~ ".$plans->price."~ "."COP ")">  <input name="test" type="text" value="1">  <input name="buyerEmail" type="text" value="[email protected]">  <input name="responseUrl" type="text" value="https://poligonourbano.soluttolabs.com/">  <input name="confirmationUrl" type="text" value="http:// www.test.com/confirmation">  <input name="Submit" type="submit" value="Enviar"></form>控制器功能：public function planView() {  $apy_key = "4Vj8eK4rloUd272L48hsrarnUA";  $plans = Plan::findOrFail(2);  return view('plans', compact('plans', 'apy_key'));}