On the HHVM blog today a new post talks about some of the work they've been doing to introduce one of the common PHP extensions, MySQLi, into the HHVM system. The post walks you through some of the process the author followed to work up the implementation.
To prepare for what was to be my big project, I rewrote the ini parser to better match Zend. [...] After warming up with the parser, I was ready to start my big project: implement MySQLi. This has been a long requested feature for HHVM. And, this extension is required to help meet our compatibility goals.
He walks you through some of the preparation steps for the work integrating the extension and the tools used for these initial steps. He briefly steps through the actual implementation and the testing of the feature (and some changes made to allow the tests to run faster). He mentions a few roadblocks hit along the way, the current status of the effort (182 passing tests, 114 failing) and some of the missing pieces yet to be worked.