public function update()
    {
        $sql = 'UPDATE ' . static::$table . ' SET ';
        $fields = get_object_vars($this);
        $data = array();
        foreach ($fields as $name => $value) {
            if ($name == 'id') {
                continue;
            }
            $sql = $sql . $name . '=:' . $name . ' ';
            $data[$name] = $value;
        }
        $sql = $sql . 'WHERE id = ' . $this->id;
        $db = new Db();
        $db->execute($sql, $data);
    }

$fields = ['field1 = :field1', 'field2 = :field2', 'field3 = :field3'];
echo implode(', ', $fields);

$fields = ['field1 = ?', 'field2 = ?', 'field3 = ?'];
echo implode(', ', $fields);