<?php

$message = 'GET /foo/bar?arg1=aaaa&arg2=bbbb';
$secret = 'secret';

$signature = base64_encode(hash_hmac('sha256', $message, $secret, true));

echo $signature;
# 82LHSJc4h/5BpLrBcGyWjTIiLuJTBYkgyGAb7cNmXew=