How does PHP determine the character encoding in browser POST request data?

∟Reference Citations in 2016

∟How does PHP determine the character encoding in browser POST request data?

'Basic Rules of Receiving Non-ASCII Characters from Input Forms' tutorial was cited in a stackoverflow forum post in 2016.

The Basic Rules of Receiving Non-ASCII Characters from Input Forms tutorial was cited in a stackoverflow forum post in 2016.

Subject: How does PHP determine the character encoding in browser POST
   request data?
Date: Sep 8, 2016
Author: the_velour_fog
Source: http://stackoverflow.com/questions/39381173
   /how-does-php-determine-the-character-encoding-in
   -browser-post-request-data

...

In regard to GET data, the W3C standard states
   Note. The "get" method restricts form data set values to ASCII
   characters.

Only the "post" method (with enctype="multipart/form-data") is
specified to cover the entire [ISO10646] character set.

So with GET the browser seems to be locked into ASCII, if the form
element has the attribute enctype="multipart/form-data" it seems the
standard supports the larger charset [ISO10646].
And I guess because it is closer to a pure bitstream, the default
Content-type of application/x-www-form-url-encoded supports all
character encodings. in particular this article states:
https://www.herongyang.com/PHP/Non-ASCII-Form-Basic-Rules.html
   URL encoding converts all non ASCII bytes in the form of "%xx", "xx"
   is the HEX value of the byte.

So this seems to explain what charsets the browser can possibly
send, but not how it instructs PHP as to what actual charset it
sent. (with the exception of GET, which PHP will know can only be
ASCII). O Other wise from what I can understand there is essentially
no direct guidance from the browser as to the character encoding of
the form data it's sending.
...