src/Type/Definition/QueryPlan.php
private function arrayMergeDeep(array $array1, array $array2) : array
    {
        $merged = $array1;

        foreach ($array2 as $key => & $value) {
            if (is_numeric($key)) {
                if (! in_array($value, $merged, true)) {
                    $merged[] = $value;
                }
            } elseif (is_array($value) && isset($merged[$key]) && is_array($merged[$key])) {
                $merged[$key] = $this->arrayMergeDeep($merged[$key], $value);
            } else {
                $merged[$key] = $value;
            }
        }

        return $merged;
    }
private function arrayMergeDeep(array $array1, array $array2) : array
    {
        $merged = $array1;

        foreach ($array2 as $key => & $value) {
            if (is_numeric($key)) {
                if (! in_array($value, $merged, true)) {
                    $merged[] = $value;
                }
            } elseif (is_array($value) && isset($merged[$key]) && is_array($merged[$key])) {
                $merged[$key] = $this->arrayMergeDeep($merged[$key], $value);
            } else {
                $merged[$key] = $value;
            }
        }

        return $merged;
    }
similar to array_merge_recursive this merges nested arrays, but handles non array values differently
while array_merge_recursive tries to merge non array values, in this implementation they will be overwritten

@see https://stackoverflow.com/a/25712428

@param mixed[] $array1
@param mixed[] $array2

@return mixed[]