By default, PHP notices are turned off and are not displayed. Nevertheless, if you can see notices in your site, you can tell PHP to hide it.
To turn this off, you can add this code to you PHP file, somewhere at the beginning of your code, e.g. to index.php:
<?php error_reporting(E_ERROR | E_WARNING | E_PARSE);
This will tell PHP to only report errors, warnings and compile-time parse errors.