[php mode] Adding new built-in keywords/functions

Updating the PHP mode with new functions/keywords (PHP 5.5).

Added:

* `yield` keyword
* `insteadof` keyword
* `__TRAIT__` constant
* `http_response_code` function
* `get_declared_traits` function
* `getimagesizefromstring` function
* `socket_import_stream` function
* `stream_set_chunk_size` function
* `trait_exists` function
* `header_register_callback` function
* `class_uses` function
* `session_status` function
* `session_register_shutdown` function