I'm comparing behavior of `!` and `empty()`, find an undocumented behavior here.
just like cast-to-boolean, `empty()` cares about if SimpleXML object is made from emty tags.
`empty(simplexml_load_string('<root />'))` is TRUE
this behavior begins from php 5.1.0~7, in 5.0.x that's FALSE
proof here => https://3v4l.org/74Tc4