Checksum Generation and Validation

$SECRETKEY = <Secret Key>;
$postData = array(
  "APPID" => <APP_ID>,
  "ORDER_ID" => <ORDER_ID>,
  "TXN_AMOUNT" => <TXN_AMOUNT>,
  "TXN_CURRENCY" => <TXN_CURRENCY>,
  "CUST_NAME" => <CUST_NAME>,
  "CUST_MOBILE" => <CUST_MOBILE>,
  "CUST_EMAIL" => <CUST_EMAIL>,
  "CUST_ADDRESS_LINE1" => <CUST_ADDRESS_LINE1>,
  "CUST_ADDRESS_LINE2" => <CUST_ADDRESS_LINE2>,
  "CUST_ADDRESS_CITY" => <CUST_ADDRESS_CITY>,
  "CUST_ADDRESS_STATE" => <CUST_ADDRESS_STATE>,
  "CUST_ADDRESS_COUNTRY" => <CUST_ADDRESS_COUNTRY>,
  "CUST_ADDRESS_POSTAL_CODE" => <CUST_ADDRESS_POSTAL_CODE>,
  "CALLBACK_URL" => <CALLBACK_URL>
);
ksort($postData);
 $signatureData = "";
 foreach ($postData as $key => $value){
  $signatureData .= $key.$value;
 }
 $signatureData;
$signature = hash_hmac('sha256', $signatureData, $SECRETKEY,true);
$signature = base64_encode($signature);