Solved the problem by using isset for each error at the beginning of the page:
PHP Code:
if (!isset($_POST['rthk'])) { $_POST['rthk'] = ""; }
Another way is to change
error_reporting = in
xampp\php\php.ini
But maybe a better way is to leave it in php.ini at the default
error_reporting = E_ALL | E_STRICT and and use on your page i.e.
error_reporting("E_COMPILE_ERROR|E_ERROR|E_CORE_ERROR");
I am aware that it's better to have all errors reported and just write better code.