'desculpe, ocorreu um erro' ao integrar o gateway de pagamento PayUMoney no modo Teste

Estou tentando integrarPayUMoney gateway de pagamento no site do meu cliente. Meu cliente me forneceu os detalhes de login dePayUMoney. Eu consegui localizar oChave do comerciante mas não consegue encontrar nenhum dos dois:

Sal mercanteSite do desenvolvedor onde posso criar uma conta de teste e testar na sandbox

PayUMoney me forneceu um formulário com algum valor padrão, mas quando testo, ele fornece a mensagem: desculpe, ocorreu um erro.

Aqui está o formulário:

<?php
// Merchant key here as provided by Payu
$MERCHANT_KEY = "JBZaLc";

// Merchant Salt as provided by Payu
$SALT = "GQs7yium";

// End point - change to https://secure.payu.in for LIVE mode
$PAYU_BASE_URL = "https://test.payu.in";

$action = '';

$posted = array();
if(!empty($_POST)) {
//print_r($_POST);
foreach($_POST as $key => $value) {    
$posted[$key] = $value; 

}
}

$formError = 0;

if(empty($posted['txnid'])) {
// Generate random transaction id
$txnid = substr(hash('sha256', mt_rand() . microtime()), 0, 20);
} else {
$txnid = $posted['txnid'];
}
$hash = '';
// Hash Sequence
$hashSequence =     "key|txnid|amount|productinfo|firstname|email|udf1|udf2|udf3|udf4|udf5|udf6|udf7|udf8|udf9|udf10";
if(empty($posted['hash']) && sizeof($posted) > 0) {
if(
      empty($posted['key'])
      || empty($posted['txnid'])
      || empty($posted['amount'])
      || empty($posted['firstname'])
      || empty($posted['email'])
      || empty($posted['phone'])
      || empty($posted['productinfo'])
      || empty($posted['surl'])
      || empty($posted['furl'])
      || empty($posted['service_provider'])
) {
$formError = 1;
} else {
//$posted['productinfo'] = json_encode(json_decode('[{"name":"tutionfee","description":"","value":"500","isRequired":"false"},{"name":"developmentfee","description":"monthly tution     fee","value":"1500","isRequired":"false"}]'));
$hashVarsSeq = explode('|', $hashSequence);
$hash_string = '';  
foreach($hashVarsSeq as $hash_var) {
  $hash_string .= isset($posted[$hash_var]) ? $posted[$hash_var] : '';
  $hash_string .= '|';
}

$hash_string .= $SALT;


$hash = strtolower(hash('sha512', $hash_string));
$action = $PAYU_BASE_URL . '/_payment';
}
} elseif(!empty($posted['hash'])) {
$hash = $posted['hash'];
$action = $PAYU_BASE_URL . '/_payment';
}
?>
<html>
<head>
<script>
var hash = '<?php echo $hash ?>';
function submitPayuForm() {
  if(hash == '') {
    return;
  }
  var payuForm = document.forms.payuForm;
  payuForm.submit();
}
</script>
</head>
<body onLoad="submitPayuForm()">
<h2>PayU Form</h2>
<br/>
<?php if($formError) { ?>
  <span style="color:red">Please fill all mandatory fields.</span>
  <br/>
  <br/>
<?php } ?>
<form action="<?php echo $action; ?>" method="post" name="payuForm">
  <input type="hidden" name="key" value="<?php echo $MERCHANT_KEY ?>" />
  <input type="hidden" name="hash" value="<?php echo $hash ?>"/>
  <input type="hidden" name="txnid" value="<?php echo $txnid ?>" />
  <table>
    <tr>
      <td><b>Mandatory Parameters</b></td>
    </tr>
    <tr>
      <td>Amount: </td>
      <td><input name="amount" value="<?php echo (empty($posted['amount'])) ? '' : $posted['amount'] ?>" /></td>
      <td>First Name: </td>
      <td><input name="firstname" id="firstname" value="<?php echo (empty($posted['firstname'])) ? '' : $posted['firstname']; ?>" /></td>
    </tr>
    <tr>
      <td>Email: </td>
      <td><input name="email" id="email" value="<?php echo (empty($posted['email'])) ? '' : $posted['email']; ?>" /></td>
      <td>Phone: </td>
      <td><input name="phone" value="<?php echo (empty($posted['phone'])) ? '' : $posted['phone']; ?>" /></td>
    </tr>
    <tr>
      <td>Product Info: </td>
      <td colspan="3"><textarea name="productinfo"><?php echo (empty($posted['productinfo'])) ? '' : $posted['productinfo'] ?></textarea></td>
    </tr>
    <tr>
      <td>Success URI: </td>
      <td colspan="3"><input name="surl" value="<?php echo (empty($posted['surl'])) ? '' : $posted['surl'] ?>" size="64" /></td>
    </tr>
    <tr>
      <td>Failure URI: </td>
      <td colspan="3"><input name="furl" value="<?php echo (empty($posted['furl'])) ?     '' : $posted['furl'] ?>" size="64" /></td>
    </tr>

    <tr>
      <td>Service Provider: </td>
      <td colspan="3"><input name="service_provider" value="<?php echo (empty($posted['service_provider'])) ? '' : $posted['service_provider'] ?>" size="64" /></td>
    </tr>

    <tr>
      <td><b>Optional Parameters</b></td>
    </tr>
    <tr>
      <td>Last Name: </td>
      <td><input name="lastname" id="lastname" value="<?php echo (empty($posted['lastname'])) ? '' : $posted['lastname']; ?>" /></td>
      <td>Cancel URI: </td>
      <td><input name="curl" value="" /></td>
    </tr>
    <tr>
      <td>Address1: </td>
      <td><input name="address1" value="<?php echo (empty($posted['address1'])) ? '' : $posted['address1']; ?>" /></td>
      <td>Address2: </td>
      <td><input name="address2" value="<?php echo (empty($posted['address2'])) ? '' : $posted['address2']; ?>" /></td>
    </tr>
    <tr>
      <td>City: </td>
      <td><input name="city" value="<?php echo (empty($posted['city'])) ? '' : $posted['city']; ?>" /></td>
      <td>State: </td>
      <td><input name="state" value="<?php echo (empty($posted['state'])) ? '' : $posted['state']; ?>" /></td>
    </tr>
    <tr>
      <td>Country: </td>
      <td><input name="country" value="<?php echo (empty($posted['country'])) ? '' : $posted['country']; ?>" /></td>
      <td>Zipcode: </td>
      <td><input name="zipcode" value="<?php echo (empty($posted['zipcode'])) ? '' : $posted['zipcode']; ?>" /></td>
    </tr>
    <tr>
      <td>UDF1: </td>
      <td><input name="udf1" value="<?php echo (empty($posted['udf1'])) ? '' : $posted['udf1']; ?>" /></td>
      <td>UDF2: </td>
      <td><input name="udf2" value="<?php echo (empty($posted['udf2'])) ? '' : $posted['udf2']; ?>" /></td>
    </tr>
    <tr>
      <td>UDF3: </td>
      <td><input name="udf3" value="<?php echo (empty($posted['udf3'])) ? '' : $posted['udf3']; ?>" /></td>
      <td>UDF4: </td>
      <td><input name="udf4" value="<?php echo (empty($posted['udf4'])) ? '' : $posted['udf4']; ?>" /></td>
    </tr>
    <tr>
      <td>UDF5: </td>
      <td><input name="udf5" value="<?php echo (empty($posted['udf5'])) ? '' : $posted['udf5']; ?>" /></td>
      <td>PG: </td>
      <td><input name="pg" value="<?php echo (empty($posted['pg'])) ? '' : $posted['pg']; ?>" /></td>
    </tr>
    <tr>
      <?php if(!$hash) { ?>
        <td colspan="4"><input type="submit" value="Submit" /></td>
      <?php } ?>
    </tr>
  </table>
</form>
</body>
</html>

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