Bug report

When extending an array with: $arr['index'] = $value; and then using array_key_exists() to check for other indexes is flagged as false as PHPStan only thinks the new 'index' exists.

Code snippet that reproduces the problem

https://phpstan.org/r/957f0c1a-0167-411e-9bdb-b548a19dd8b6

Expected output

There shouldn't be any error.