at line 54
replace
                $this->db_connection = new PDO('mysql:host='. DB_HOST .';dbname='. DB_NAME . ';charset='.DB_CHARSET, DB_USER, DB_PASSWORD);

with:

                $this->db_connection = new PDO('mysql:host='. DB_HOST .';dbname='. DB_NAME . ';charset=utf8', DB_USER, DB_PASSWORD);