db::sum PHP Method

    static function sum($table, $column, $where = null)
    {
        $sql = 'SELECT SUM(' . $column . ') AS sum FROM ' . self::prefix($table);
        if (!empty($where)) {
            $sql .= ' WHERE ' . self::where($where);
        }
        $result = self::query($sql, false);
        $result = self::fetch($result);
        return a::get($result, 'sum', 0);
    }

 public function testSum()
 {
     $this->assertEquals(6, db::sum('users', 'id'));
 }