According to http://php.net/manual/en/function.urldecode.php, PHP does urldecode() on $_GET and on $_REQUEST (which contains $_POST).
But is directly calling $_POST already decoded?
Yes, all the parameters you access via $_GET and $_POST are decoded.
The reason the urldecode() documentation doesn't mention $_POST is because the POST data might not be URL-encoded in the first place. It depends on whether the POST data is submitted in application/x-www-form-urlencoded format or multipart/form-data format.
But all this is transparent to the application.
The documentation of $_GET does mention this explicitly, though.
Note:
The GET variables are passed throughurldecode().