mobiki is a small Wiki-engine implemented in PHP. It is optimized for usage with mobile devices, slim HTTP requests and fast HTML5 content delivery. It will stay small, understandable and easy to hack - no bells, no whistles - just tin whistles.
mobiki is really small, just a few kilobytes. Open http://test.datenbrei.de to see, how it shows up on your mobile phone. You can edit pages from your phone as easy as from your computer - both work. Even there is just one stylesheet, mobiki works on any output device and also has an extra print output showing just the content. It has a very reduced feature set, but still supports search, hashtags and provides a RSS feed for use with aggregators. It's more like a smallish CMS.