PHP-RQL fails badly if mbstring.func_overloading is enabled

[danielmewes]

See https://groups.google.com/forum/#!topic/rethinkdb/d4WqBxNDaXQ and http://php.net/manual/en/mbstring.overload.php

I don't think there is much we can do. This overloading just completely changes the semantics of the PHP string functions which we rely on for processing binary data.

What we probably should do is to check whether MB_OVERLOAD_STRING is set and to fail somewhat more gracefully with an error message