class DataBase {

    public static $db = null;
    public $msqli;

    public static function getDb() {
        if (self::$db == null) {
            self::$db = new DataBase();
        }
        return self::$db;
    }

    private function __construct() {
        $this->msqli = mysqli_connect('localhost', 'root', '', 'depsoft');

    }

    public function __destruct() {
        if ($this->msqli) {
            $this->msqli->close();
        }
    }
}

require_once "connect.php";
class DateFormToInsert extends DataBase {

    public static function dateFromToInsert() {

        $query_user = mysqli_query($this->msqli, "SELECT * FROM `region_table`");
        $data = mysqli_fetch_array($query_user, MYSQLI_ASSOC);
        echo "<pre>";
        print_r($data);
    }
}

$aa = DateFormToInsert::getDb();
DateFormToInsert::dateFromToInsert();

class DB
{
    private static $_instance = null;

    // для безопасности настройки лучше хранить в файле с конфигом
    private static DB_HOST = '';
    private static DB_NAME = '';
    private static DB_USER = '';
    private static DB_PASS = '';

    private function __construct () {

        $this->_instance = new PDO(
            'mysql:host=' . self::DB_HOST . ';dbname=' . self::DB_NAME,
            self::DB_USER,
            self::DB_PASS,
            [PDO::MYSQL_ATTR_INIT_COMMAND => "SET NAMES 'utf8'"]
        );

    }

    private function __clone () {}
    private function __wakeup () {}

    public static function getInstance()
    {
        if (self::$_instance != null) {
            return self::$_instance;
        }

        return new self;
    }
}


require_once "connect.php";
class DateFormToInsert  {

    public static function dateFromToInsert() {

        $db = DB::getInstance();

        $query_user = $db->prepare("SELECT * FROM `region_table`")->execute();
        $result = $sth->fetchAll();
        print_r($result);
    }
}

$query_user = mysqli_query(Database::getDb(), "SELECT * FROM `region_table`");