[php-src] Issue #8906: wish: config option to make undefined array key a notice again

Issue: https://github.com/php/php-src/issues/8906 Author: RichardNeill ### Description In PHP 7, underfined array keys created E_NOTICE. This is now E_WARNING. Can we please have a configuration option to put that back to E_NOTICE? Rightly or wrongly, there are a lot of people, myself included, who have thousands of lines of code with things such as: `` if ($_GET['xxx']) `` #where xxx may or may not exist. or ``if ($_GET['xxx'] == 'yyy')`` and in PHP8, we now get flooded with warnings "Undefined array key", which get in the way of real warnings. Migrating and testing this code will take a long time, and furthermore, it's really ugly and harder to read: ``if (isset($_GET['xxx'))`` or ``if (isset($_GET['xxx']) && $_GET['xxx'] == 'yyy'))`` to suppress this warning. Instead, it would be better to have some sort of pragma in the main header ``ini_set("array_key_missing_enotice", true);`` ``ini_set ("error_reporting", E_ALL & ~E_NOTICE);`` To me, it seems sensible that a defined associative array, with a key that isn't present (and may not be expected), should not be a warning, especially if it's being tested for with if(). It may also be worth treating undefined variables the same way. Thanks for your help,