In-app purchase signature verification with PHP openssl

问题

In an attempt to follow some of the security guidelines for in-app purchase here: http://developer.android.com/guide/market/billing/billing_best_practices.html I am trying to do signature validation on a server instead of in the app iteself. I would ideally like to use the php openssl libraries and it looks like code such as the following should work:

$public_key_str = file_get_contents("./pubKey/out");
$public_key_str = trim($public_key_str);
$key = openssl_get_publickey($public_key_str);
if(!$key)
{
echo 'Can\'t get public key';
}
$signature = base64_decode( $signature );
$ok = openssl_verify($data, $signature, $key);
var_dump($ok);

I know both my signature and public key are correct, but $ok is 0! Signature I try to use is string from the app purchase bundle. Guess my key is correct and the problem is in signature. When I try to decode it from base64 with: openssl enc -base64 -d -in signature -A > signature.bin, I have the string same with base64_decode(). Any ideas ?

UPD: also I don't really understanf what I should pass in openssl_verify() as $data ? My data looks like this:

$data = '{"nonce":5550262978898439313,"orders":[{"notificationId":"android.test.purchased","orderId":"transactionId.android.test.purchased","packageName":"com.ads.testbilling","productId":"android.test.purchased","purchaseTime":1308224646237,"purchaseState":0}]}';