public function query($query) {
    $this->last_query = $query;
    $resultado = mysqli_query($this->connection, $query);
    $this->confirm_query($resultado);
    return $resultado;
}

private function confirm_query($result) {
    if ($result  === FALSE) {
        print_r(mysqli_fetch_assoc($result));
        $output = "Error on query".  mysqli_error($this->connection)."<br/>";
        $output .= "Last query : ".$this->last_query;
        die($output);
    }
}

public function fetch_array($result_set){

    return mysqli_fetch_array($result_set);

}

public static function find_by_id($id=0){
    global $database;
    $result_set = $database -> query("SELECT * FROM users WHERE id={$id}");
    $found = $database ->fetch_array($result_set);
    return $found;
}

$result_set = User::find_by_id(1);
$found_user = $database ->fetch_array($result_set);