I read from the database and write again the text in Russian. Result - ???. What can be wrong?

Question

Character encoding

I do it like this:

$rez_products = $parser_mysqli->query('
  SELECT * 
  FROM `'.CATS_TABLE.'` 
  WHERE 1
');
while($link = $rez_products->fetch_object())
{
  //echo $link->name.'<br>';
  var_dump($parser_mysqli->query('
    UPDATE `'.CATS_TABLE.'` 
    SET `name` = "'.$link->name.'"
    WHERE id = "'.$link->id.'"
  '));
}

As a result, there are question marks in the name field in the database. What can be wrong? I guess it's in encodings, but what exactly?