It should be possible to "extend" (of) Arrays. Use case:

/**
 * @template T of array
 * @param T $arr
 * @return T & array{mykey: int}
 */
function addSomeKey(array $arr, int $value): array {
	$arr['mykey'] = $value;
	return $arr;
}

Playground: https://phpstan.org/r/2a391b38-912d-4ea5-a71a-a1fab833d2f8

Right now, this produces multiple errors, the root problem being:

PHPDoc tag @template T for function addSomeKey() with bound type array is not supported.