$btc = "0.01000000";
$call = $api->call('sendfrom', $sender_account, $receiving_address, $btc,6);
it just returns null, so i have no idea what the error is.
I'm using the testnet.
Make sure $btc is a numeric value rather than a string, in php;
$call = $api->call('sendfrom', $sender_account, $receiving_address, $btc,6);
Your confirmation number may also need to be an int (e.g. intval("6") )