Generate a data Signature using PHP and OPENSSL

RsaSignature.php

<?php

$priv_key = file_get_contents("./key.pem");

$pkeyid = openssl_get_privatekey($priv_key);

$date_time="2016-06-01 12:00:00";
$amount="1000";
$narrative="Test transaction";
$network_ref="1000100";
$external_ref="ndhund-dkjndh-ldkjnd-ppolkiuj";
$msisdn="256777000999";

$data = $date_time . $amount . $narrative . $network_ref . $external_ref . $msisdn;

$signature = "";

openssl_sign($data, $signature, $pkeyid);

openssl_free_key($pkeyid);

// Base64 RSA signature
$signature = base64_encode($signature);

echo $signature;
echo "\n";