Convert multidimensional array to simple array with keys

php_array_flatten.md

flatten(array $array, $keyPrefix = ''): array
{
    $return = [];

    foreach ($array as $key => $value) {
        $key = trim($keyPrefix . '_' . $key, '_');

        if (is_array($value)){
            $return = array_merge($return, flatten($value, $key));
        } else {
            $return[$key] = $value;
        }
    }

    return $return;
}