สอบถามเรื่อง การ Close Connection ของ MySQLi ใน Function ครับ

<?php
Class MyDB {

  protected $_DB_HOST = 'localhost';
  protected $_DB_USER = 'root';
  protected $_DB_PASS = 'root_password';
  protected $_DB_NAME = 'my_sql';
  protected $_conn;

  public function __construct() {
      $this->_conn = mysqli_connect($this->_DB_HOST, $this->_DB_USER, $this->_DB_PASS);
      if($this->_conn) {
        $this->_conn->set_charset("utf8");
          //echo 'We are connected!<br>';
      }
  }

  public function connect() {
      if(!mysqli_select_db($this->_conn, $this->_DB_NAME)) {
          die("1st time failed<br>");
      }

      return $this->_conn;
  }
}

?>

<?php
Class Member {
  protected $_conn;

  public function __construct() {
      $db = new MyDB();
      $this->_conn = $db->connect();
  }

  public function CheckUserID($value)
  {
    $result = $this->_conn->query("SELECT * FROM member WHERE UserID ='".$value."'");
    if(!$result) {
        return false;
        // return mysqli_errno($this->_conn) . mysqli_error($this->_conn);
    }
    if ($result->num_rows > 0) {
      return true;
    }
    return false;
    $this->_conn->close();
  }
?>