Description:
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input_encoding, internal_encoding, output_encoding are not writable on Win32.
Documentation says that those values are PHP_INI_ALL and, on other OS (alexdupre checked it on FreeBSD), it actually is.
On Windows it seems not.

I had the doubt that "the OS didn't support that specific charset", but this can't be as "string iconv ( string in_charset, string out_charset, string str)" works perfectly with the very same charset names.

Reproduce code:
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All of them die:

iconv_set_encoding('internal_encoding', 'Windows-1252') or die('Cannot change internal encoding.');
iconv_set_encoding('output_encoding', 'UTF-8') or die('Cannot change output encoding.');
ini_set('iconv.internal_encoding', 'Windows-1252') or die('Cannot change internal encoding.');
ini_set('iconv.output_encoding', 'UTF-8') or die('Cannot change output encoding.');

Expected result:
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A change in charset.

Actual result:
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A return value of FALSE.