public static function cells() { if (!isset(self::$cells)) { self::$cells = new collection(); foreach (self::$cell_codes as $code) { $cell = new lat_lng_bound(geometry::os_to_lat_long($code . '99999999'), geometry::os_to_lat_long($code . '000000')); $cell->code = $code; self::$cells[] = $cell; } } return self::$cells; }