>>> 'abcd'.encode('ascii')
'abcd'
And decode unicode:
>>> u'abcd'.decode("ascii")
u'abcd'
It's nonsensical.
Thankfully Python 3 has removed all this madness:
>>> b'abcd'.encode('ascii')
AttributeError: 'bytes' object has no attribute 'encode'
>>> 'abcd'.decode('ascii')
AttributeError: 'str' object has no attribute 'decode'
(For those not familiar, Python 3 switched around the notation for unicode/bytes. In Python 2 "abcd" is a bytes literal, adding u makes it unicode, in Python 3 "abcd" is a unicode literal, adding b makes it bytes.)
PHP would deal with uploaded files by itself and write them correctly and directly to disk (unlike some Java implementations like Nexus which buffers in RAM, you can guess what happens).
As for ordinary POST/GET parameters, it stores them in a string aka a byte stream which you can then post-process e.g. by translating to UTF-8 based on the browser encoding header. So basically the only way to shoot yourself is if you're doing substr and friends on user input instead of using the mb_ variants.
unlike some Java implementations like Nexus which buffers in
RAM, you can guess what happens
PHP, generally speaking, doesn't do Unicode at all. Outside of functions which explicitly do encoding conversions (mbstring, iconv, etc), all "strings" are just handled as a bag of bytes.
The main footguns I'm aware of are the "utf8_encode" and "utf8_decode" functions, which actually do lossy UTF8 <-> ISO8859-1 conversions.