Coinbase Exchange signing with PHP

gistfile1.php

<?php
class CoinbaseExchange {
    public function __construct($key, $secret, $passphrase) {
        $this->key = $key;
        $this->secret = $secret;
        $this->passphrase = $passphrase;
    }

    public function signature($request_url='', $body='', $timestamp=false, $method='GET') {
        $body = is_array($body) ? json_encode($body) : $body;
        $timestamp = $timestamp ? time() : $timestamp;

        $what = $timestamp.$method.$request_url.$body;

        return base64_encode(hash_hmac("sha256", $what, base64_decode($this->secret), true));
    }
}