Signature bug

[bispldeveloper]

Hi

I have been trying to get the "Cybersource" signature using the below codes but the response always says - "authentiaction failed!"
Can you please help me to solve this

/*Signature code/
$signatureString = "host: ".$request_host."\nrequest-target: ".$httpMethod." ".$resourcePath."\ndigest: SHA-256=".$digest."\nv-c-merchant-id: ".$merchant_id;
$signatureByteString = utf8_encode($signatureString);
$decodeKey = base64_decode($merchant_secret_key);
$signature = base64_encode(hash_hmac("sha256", $signatureByteString, $decodeKey, true));