gistfile1.php

<?php
function parse_signed_request($signed_request) {
    list($encoded_sig, $payload) = explode('.', $signed_request, 2); 
    
    // decode the data
    $sig = base64_url_decode($encoded_sig);
    $data = json_decode(base64_url_decode($payload), TRUE);
    
    // confirm the signature
    $expected_sig = hash_hmac('sha256', $payload, $secret, $raw = TRUE);
    
    if ($sig !== $expected_sig) {
        error_log('Bad Signed JSON signature!');
        return NULL;
    }
    
    return $data;
}

function base64_url_decode($input) {
    return base64_decode(strtr($input, '-_', '+/'));
}


$data = parse_signed_request($_REQUEST['signed_request']);

response.txt

{
   "oauth_token": "{user-access-token}",
   "algorithm": "HMAC-SHA256",
   "expires": 1291840400,
   "issued_at": 1291836800,
   "user_id": "218471"
}