Fix #51903: simplexml_load_file() doesn't use HTTP headers

The `encoding` attribute of the XML declaration is optional; it is good
practice to use external encoding information where ava

Fix #51903: simplexml_load_file() doesn't use HTTP headers

The `encoding` attribute of the XML declaration is optional; it is good
practice to use external encoding information where available if it is
missing. Thus, we check for `charset` info of `Content-Type` headers,
and see whether the encoding is supported.

We cater to trailing parameters and quoted-strings, but not to escaped
backslashes and quotes in quoted-strings, since no known character
encoding contains these anyway.

Co-authored-by: Michael Wallner <mike@php.net>

Closes GH-6747.

show more ...