[php-src] Issue #8654: Create array element by simply incrementing it

Issue: https://github.com/php/php-src/issues/8654 Author: brambil ### Description Scenario: I have a multilevel array $arr dinamically populated (p.e. from MySQL queries) that I use to count. The multilevel array is generated in this way: $arr[$index1][$index2]++ That code generates warnings when in $arr[$index1][$index2] one of $index1 or $index2 aren't already defined, so it's necessary use an if statement to check the existence before use it. Input: remove the warning, or convert to a less relevant notice.