the code in php to generate a signature  is like this

<?php

function generateDokuSignature(

    string $clientId, 

    string $sharedKey, 

    string $requestTimestamp, 

    string $requestTarget, 

    string $digest

): string {

    // 1. Concatenate the components in the exact order required by DOKU

    // Formula: Client-Id + Request-Timestamp + Request-Target + Digest

    $signatureComponent = $clientId . $requestTimestamp . $requestTarget . $digest;

    // 2. Generate HMAC-SHA256 hash using the Shared Key

    // The third parameter is the key (Shared Key), the second is the data (Component)

    $rawSignature = hash_hmac('sha256', $signatureComponent, $sharedKey, true);

    // 3. Convert the binary hash to a Hexadecimal string

    $finalSignature = bin2hex($rawSignature);

    return $finalSignature;

}

// ==========================================

// USAGE EXAMPLE

// ==========================================

// 1. Your Credentials (Get these from DOKU Dashboard -> Settings -> Integration)

 $client_id = "BRN-0225-xxxxxx"; 

 $shared_key = "SK-xxxxxxxxx"; 

// 2. Request Details

// Timestamp must be ISO 8601 format (UTC) or as required by specific endpoint

 $request_timestamp = "2023-11-01T10:00:00Z"; 

// The API Path you are calling (e.g., for Checkout Page)

 $request_target = "/checkout-pages/v1/payment"; 

// The Digest you calculated from the JSON Body (Base64(SHA256(Body)))

// Example Digest: "X4kZ8v..." (This is just a placeholder, use your actual calculation)

 $request_digest = "X4kZ8v..."; 

// 3. Generate the Signature

 $signature = generateDokuSignature(

    $client_id,

    $shared_key,

    $request_timestamp,

    $request_target,

    $request_digest

);

echo "Generated Signature: " . $signature;

?>

what function to use t o generate HMAC-SHA256 hash ?

thanks