مشکل در کارکترست utf-8

سلام.

من یک کد دارم اما وقتی به دیتابیس کانکت میشم علامت سوال نشون میده

چطور به utf8 تغییر بدم؟؟

 $db_username = 'root';
$db_password = '';
$db_name = 'th';
$db_host = 'localhost';
$item_per_page = 12;

$mysqli = new mysqli($db_host, $db_username, $db_password, $db_name);
//Output any connection error
if ($mysqli->connect_error) {
 die('Error : ('. $mysqli->connect_errno .') '. $mysqli->connect_error);
}
$page_number = filter_var($_POST["page"], FILTER_SANITIZE_NUMBER_INT, FILTER_FLAG_STRIP_HIGH);

//throw HTTP error if page number is not valid
if(!is_numeric($page_number)){
 header('HTTP/1.1 500 Invalid page number!');
 exit();
}

//get current starting point of records
$position = (($page_number-1) * $item_per_page);

//fetch records using page position and item per page. 
$results = $mysqli->prepare("SELECT id, name, cat FROM test ORDER BY id DESC LIMIT ?, ?");
//bind parameters for markers, where (s = string, i = integer, d = double, b = blob)
//for more info https://www.sanwebe.com/2013/03/basic-php-mysqli-usage
$results->bind_param("dd", $position, $item_per_page); 
$results->execute(); //Execute prepared Query
$results->bind_result($id, $name, $message); //bind variables to prepared statement

//output results from database
while($results->fetch()){ //fetch values
 echo '<li>'.$id.') <strong>'.$name.'</strong> : '.$message.'</li>'; 
}