It doesn't need to be written in a certain way, there may be new features in recent releases of PHP that don't work on HHVM but the HHVM folks are pretty quick to add in support for those features. Here is a little library[0] I wrote in PHP and this is tested on HHVM using Travis (see .travis.yml). I haven't had to write the PHP any differently, it just works (although it's an incredibly small library).

Edit: The speaker in the video also mentions that they have taken the test suites from the top twenty PHP projects and they test them against new releases of HHVM, further confirming that the syntax requires no special attention except for new language constructs.

[0] https://github.com/unfunco/bijective