Description:
------------
When using htmlspecialchars with a invalid multibyte string and using
UTF-8 as encoding, there are two possible outcomes based on the
"display_errors" ini setting:

1. display_errors=1
=> empty string is returned

2. display_errors=0
=> E_WARNING is thrown

This is exactly what the code states. Can be viewed in 
http://cvs.php.net/viewvc.cgi/php-src/ext/standard/html.c?view=markup
on line 1147

However this is VERY confusing as a developer point of view. As I have
display_errors always set to "1" for debugging purposes, I never
realized, one of our locale strings was corrupt, as it was just emptied
out.

Now in the production environment, our error handler terminates the
script because of the E_WARNING beeing thrown.

While both of the ways (empty string / error) are acceptable for me -
because ofcourse the input string is invalid, it is very confusing to
have different behaviors of PHP based on the display_errors setting.

Reproduce code:
---------------
echo 'a' . htmlspecialchars(substr(utf8_encode('aü'), 0, 2), ENT_QUOTES,
'UTF-8') . 'b';

Expected result:
----------------
Either 'ab'
Or PHP E_WARNING

However not both based on display_errors

Actual result:
--------------
display_errors=1 => 'ab'
display_errors=0 => E_WARNING

It's intentional. If you disagree, please ask stas@php.net why it is
like this (I once reverted that :) 

The idea is to return an error but not display it (i.e. log it or allow
custom error handlers to process it). 

The reason for it is that, unfortunately, people run servers in
production with display_errors=On, and php_escape_html_entities_ex can
be triggered from all kinds of code that usually doesn't produce errors,
which can reveal sensitive information on public sites.  So we chose to
go after lesser of two evils and not generate the error in this
context.

For debugging, I would suggest always logging errors and checking the
error log, as some errors may be hard to spot in display anyway
(especially true if your script produces something like JSON).