We'll be releasing Objective-C, Java, C# open source SDK libraries for this soon. We already have JavaScript and Python plus a command line tool (wlist) out at https://github.com/wunderlist/
How about PHP?
We aren't working on it now since we don't use PHP internally but we'd be happy to support a developer who wants to build support!
We worked with it for @Scanbot, fast implementation, well documented, worked flawlessly. Thanks 6W Team!
<3
Great looking forward to it, especially Slack Integration!