It doesn't support any of the new PHP 5.3 language features Private properties don't really exist under HipHop. They are treated as if they are protected instead. You can't unset variables. unset will clear the variable, but it will still be in the symbol table. *eval* and *create_function* are limited Dynamic defines won't work: *define($name,$value)* *get_loaded_extensions()*, *get_extension_funcs()*, *phpinfo()*, *debug_backtrace()* don't work Conditional and dynamically created include filenames don't work as you might expect You are currently limited to these extensions: Quite a few, but the lack of *mysqli*, *pgsql*, and *memcache* makes it hard to run some apps.