'rb' is a valid mode, and means Read-Only Binary Mode. This mode is used for portability reasons (see fopen).
(PHP 4 >= 4.1.0, PHP 5 >= 5.2.0, PECL zip >= 1.0.0)
zip_entry_open — Open a directory entry for reading
Opens a directory entry in a zip file for reading.
TRUE on success or
FALSE on failure.
Unlike fopen() and other similar functions, the return value of zip_entry_open() only indicates the result of the operation and is not needed for reading or closing the directory entry.