$guid="xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx";
$firstpassword="xxxxxxxxxxx";
$secondpassword="xxxxxxxxxxx";
$amounta = "600";
$addressa = "1NfGMB5DNVitjvUKk2UJw2iZH28sAGP6XX";
$recipients = urlencode('{
                  "'.$addressa.'": '.$amounta.'
               }');

$json_url = "https://blockchain.info/merchant/$guid/sendmany?password=$firstpassword&second_password=$secondpassword&recipients=$recipients";

echo $json_url."\n";

$json_data = file_get_contents($json_url);
$json_feed = json_decode($json_data);

var_export($json_feed);