|$parts = ['aa', 'aaa', ''];
|_______________________
|count = 3
|$n = 2
|echo = aaa
|~~~~~~~~~~~~~~
|$parts = ['aa', 'aaa'];
|_______________________
|count = 2
|$n = 1
|echo = aaa
|~~~~~~~~~~~~~~
|$parts = [];
|_______________________
|count = 0
|$n = -1
|echo =
|~~~~~~~~~~~~~~