Which canonical PHP.net implementation? They differ across minor revisions, in intentional and unintentional ways. In the absence of a standard, saying what is

Which canonical PHP.net implementation? They differ across minor revisions, in intentional and unintentional ways. In the absence of a standard, saying what is and is not PHP is necessarily a practical matter: useful PHP implementations are those that run non-trivial PHP applications. HipHop qualifies. FB's dependence on HipHop is because of backwards-compatible extensions to the language (like yield, e.g.). These extensions don't prevent HipHop from running normal PHP programs, though our use of them does prevent us from using Zend. The extensions are under flags that default off, if you don't want to use them.