This bug has been fixed in SVN.

Snapshots of the sources are packaged every three hours; this change
will be in the next snapshot. You can grab the snapshot at
http://snaps.php.net/.
 
Thank you for the report, and for helping us make PHP better.

This bug has been only fixed on trunk, because from what I can see, it might cause BC issue on 5.3 series.

Hello,

In version 5.3.3 and 5.3.4, the problem is still not
resolved

Yes, as I mentioned, the fix was committed on trunk, not in 5.3 branch.

Ok, sorry

I don't think this bug was fixed correctly. In particular, iconv doesn't handle '//IGNORE' correctly now.

ezyang@javelin:~$ cat test.php
<?php var_dump(iconv("UTF-8", "ISO-8859-1//IGNORE", "\xE4\xB8\xAD"));
ezyang@javelin:~$ php --version
PHP 5.3.6-13ubuntu3.3 with Suhosin-Patch (cli) (built: Dec 13 2011 18:37:10) 
Copyright (c) 1997-2011 The PHP Group
Zend Engine v2.3.0, Copyright (c) 1998-2011 Zend Technologies
    with Xdebug v2.1.0, Copyright (c) 2002-2010, by Derick Rethans
ezyang@javelin:~$ php test.php
PHP Notice:  iconv(): Detected an illegal character in input string in /home/ezyang/test.php on line 1
PHP Stack trace:
PHP   1. {main}() /home/ezyang/test.php:0
PHP   2. iconv() /home/ezyang/test.php:1
string(0) ""
ezyang@javelin:~$ phpv 5.4.0RC3 --version
PHP 5.4.0RC3 (cli) (built: Dec 16 2011 15:14:12) 
Copyright (c) 1997-2011 The PHP Group
Zend Engine v2.4.0, Copyright (c) 1998-2011 Zend Technologies
ezyang@javelin:~$ phpv 5.4.0RC3 test.php
bool(false)

This is a totally unrelated issue to what you try to explain. And you already have 
a report for this, #48147

I note that the docs still reflect the old behavior...

> I note that the docs still reflect the old behavior...

Which docs? <http://php.net/manual/en/function.iconv.php> seems to be fine.

Description:
------------
Fixed bug #52211 (iconv() returns part of string on error)
http://svn.php.net/viewvc/?view=revision&revision=300894
http://git.php.net/?p=php-src.git;a=commit;h=e3fdf31b0bf3b7b799ea25ba5ec14be194813e0b

This fix make "iconv() with //IGNORE" return false

Test script:
---------------
<?php
    $str = "\xC3\xE6\xBC\xEC\xCC\xD6\xC9\xEE\xBF";
    $str_iconv = iconv('GBK', 'UTF-8//IGNORE', $str);
    var_dump($str_iconv);
?>

Expected result:
----------------
Notice: iconv(): Detected an incomplete multibyte character in input string in %s on line %d
string(12) "\xE9\x9D\xA2\xE6\xA3\x80\xE8\xAE\xA8\xE6\xB7\xB1"

Actual result:
--------------
Notice: iconv(): Detected an incomplete multibyte character in input string in %s on line %d
bool(false)