For the most part, HHVM is quite compatible with the PHP engine. In other words, PHP code that runs on the PHP engine will run unmodified with the exact output on HHVM.
However, there are some noted inconsistencies. They are listed here by category.
Note: This list may not be exhaustive; If you are using HHVM and come across an inconsistency not listed in these docs, please report a documentation issue.