We'd love to have your help in making HipHop better. If you run into problems, please open an issue, or better yet, fork us and send a pull request.
If you want to help but don't know where to start, try fixing some of the Zend tests that don't pass. You can run them with hphp/test/run. When they work, move them to zend/good and send a pull request.
All the open issues tagged Zend incompatibility are real issues reported by the community in existing PHP code and frameworks that could use some attention.
Before changes can be accepted a Contributor Licensing Agreement (pdf - print, sign, scan, link) must be signed.
Please add appropriate test cases as you make changes; see here for more information. Travis-CI is integrated with this GitHub project and will provide test results automatically on all pulls.