default_charset and friends

Yasuo, internals, doc-friends,

I'm working on the 5.6 migration guide, and I'm a little confused
about what state the default encoding RFC actually ended up in after
it was accepted and merged. UPGRADING says:

Changes were made to character set handling in:

• the iconv and mbstring extensions,
• and htmlentities(), htmlspecialchars(), html_entity_decode() functions

The precedence for these is now:

default_charset < internal/input/output_encoding < (mbstring.* || iconv.*) < function parameter

For example, the easiest way to use the UTF-8 encoding is to set
default_charset=UTF-8 and leave the following php.ini parameters

The way this reads to me, assuming I don't have any other encoding
settings set, I should be able to set default_encoding to (for
example) "cp1252" and get Windows-1252 handling as the default in
htmlentities(), htmlspecialchars() and html_entity_decode() if I don't
specify the encoding parameter. In practice, though, that doesn't seem
to be the case. I created this script:

<?php
ini_set('default_charset', 'cp1252');

var_dump(htmlentities("\xA3", ENT_HTML5));
var_dump(htmlentities("\xA3", ENT_HTML5, 'cp1252'));

var_dump(html_entity_decode("£", ENT_HTML5));
var_dump(html_entity_decode("£", ENT_HTML5, 'cp1252'));
?>

And ran it using the -n option, so that anything I had set in my
php.ini would be ignored. My expected output would have been for the
two htmlentities() and html_entity_decode() calls to return the same
strings (ignore the unknown character glyphs; my terminal is UTF-8, so
I expect Windows-1252 output to be broken):

string(7) "£"
string(7) "£"
string(1) "�"
string(1) "�"

But instead, I got this output, suggesting that the calls without
explicit charset parameters were treated as UTF-8:

string(0) ""
string(7) "£"
string(2) "£"
string(1) "�"

What am I missing here? Do htmlentities(), htmlspecialchars() and
html_entity_decode() actually respect default_charset? (Also, if I set
internal_encoding, input_encoding and output_encoding, which one
should get used for each?)

Thanks,

Adam