$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);