Description:
------------
get_html_translation_table() returns an array with two keys having value "'" (the ampersand). When array_flip()-ed, as suggested by the manual, the second one overwrites the first, thus giving a reverse table with only one ampersand key. Using this reverse table to convert back a string is not possible, since the original table had the other representation of the ampersand first, and used that for translation.

See http://lxr.php.net/source/php-src/ext/standard/html.c#466

Note that if this is not deemed to be a PHP bug, then this irreversibility issue should be documented.

Reproduce code:
---------------
var_dump(htmlspecialchars("'", ENT_QUOTES));
var_dump(get_html_translation_table(HTML_SPECIALCHARS, ENT_QUOTES));
var_dump(array_flip(get_html_translation_table(HTML_SPECIALCHARS, ENT_QUOTES)));


Expected result:
----------------
No ambigous array elements in the translation table.

Actual result:
--------------
Two entries for the ampersand.