สอบถามเรื่อง mysqli_set_charset ต้องตั้งค่าอย่างไรให้สามารถใช้งานภาษาไทย utf-8 ได้ครับ

class Db{
		public static $_mysqli = null;
		 
		private $_host = "localhost",
				$_database = "xxxxx",
				$_username = "root",
				$_password = "root";
		public function __construct(){
			//instace of db class
			return $this->Connnect();
		}
		//method to connect to the db
		public function Connnect(){
			self::$_mysqli = mysqli_connect($this->_host,$this->_username,$this->_password,$this->_database);
			//check for connection error
			if(!mysqli_connect_errno()){
				//no error
				return self::$_mysqli;
			}else{
				echo "Connection Failed ". mysqli_connect_error();
			}
		}
}