$billerId = '9999986379225246';
$authToken = '16dfe8d7-889b-4380-925f-9c2c6ea4d930';

$auth = $billerId . ':' . $authToken;

//this results in error
$auth_key_byte_array = unpack("H*",$auth);

//this also results in error
$auth_key_byte_array = hash_hmac("sha256", $auth, false);

//even tried a loop function
function create_byte_array($string){
    $array = array();
    foreach(str_split($string) as $char){
        array_push($array, sprintf("%02X", ord($char)));
    }
    return implode('', $array);
}

$auth_key_byte_array = create_byte_array($auth);