There is also small typo in documentation but I dont want to open another bug.
On http://ie.php.net/mbstring this section is repeated twice:

Name in the IANA character set registry: UTF-16BE
Underlying character set: Unicode
Description: See above.
Additional note: In contrast to UTF-16, strings are always assumed to be in big endian form. 

While one should be about UTF-16BE and other about UTF-16LE.

I think this is correct as you are not supposed to supply a BOM if you specify which endianness your UTF16 stream is in.

I'm not specifying which endianess mb_convert_encoding should use to convert to ISO. Look:
$utf16LE2iso = mb_convert_encoding($utf16LE,'ISO-8859-1','UTF-16');

I'm converting from UTF-16 (LE or BE) to ISO-8859-1. It looks like mb_convert_encoding is checking BOM field and choosing right encoding (if you remove BOM field it won't be converted properly for one endianess). The only problem is that BOM is not ignored.

The first two lines with endianess specified:
$utf16LE = mb_convert_encoding($iso_8859_1,'UTF-16LE','ISO-8859-1');
$utf16BE = mb_convert_encoding($iso_8859_1,'UTF-16BE','ISO-8859-1'); are just for convient UTF-16 string creation, please ignore them.

ah, mbstring has a weird parameter order (dest, src) instead of (src, dest)... did you try to use iconv perhaps?

The same example but with iconv instead of mb_convert_encoding works perfect - but it doesn't close bug related to mb_convert_encoding I guess :).

Another problem exist with converting to 'UTF-16' (using mb_convert_encoding) - BOM section is not added. Again iconv works well in this case.

Please try using this CVS snapshot:

  http://snaps.php.net/php5-latest.tar.gz
 
For Windows:
 
  http://snaps.php.net/win32/php5-win32-latest.zip

No feedback was provided for this bug for over a week, so it is
being suspended automatically. If you are able to provide the
information that was originally requested, please do so and change
the status of the bug back to "Open".

There are two problems when mb_convert_encoding is 
converting from UTF-16:

1) It is including the (transcoded) BOM in the result, 
rather than stripping it

2) If the source UTF-16 string was little endian, then the 
second character of the conversion will be wrong; it is 
converted as if the character code had 0xFF00 or'd into it.

Problem 1 occurs with any UTF-16 variant (though it is 
arguably correct behavior for UTF-16LE and UTF-16BE).  
Problem 2 only occurs when converting from UTF-16.

This PHP program demonstrates this all clearly:



function dump($s)
{
	for ($i = 0; $i < strlen($s); ++$i) {
		echo substr(dechex(256+ord(substr($s, $i, 1))), 1, 
2),  ' ';
	}
	var_dump($s);
}

$utf16le = "\xFF\xFE\x41\x00\x42\x00\x43\x00";
$utf16be = "\xFE\xFF\x00\x41\x00\x42\x00\x43";
	// these strings are both valid UTF-16, the BOM at the 
start indicates
	// the endianness.  We don't expect the BOM to be 
included in a conversion

echo "The UTF-16LE and UTF-16BE sequences:\n";
dump($utf16le);
dump($utf16be);
echo "\n";

$encodings = array("ascii", "iso-8859-1", "utf-8", "utf-16", 
"utf-16le", "utf-16be");

foreach ($encodings as $enc) {
	echo "Converting to $enc:\n";
	dump(mb_convert_encoding($utf16le, $enc, "utf-16"));
	dump(mb_convert_encoding($utf16be, $enc, "utf-16"));
	echo "\n";
}

UTF-16LE and UTF-16BE seem mixed up when using mb_convert_encoding.

I want to read the content of a file in UTF-16BE (starts with \xFE\xFF) and convert it into UTF-8:

$s = file_get_contents($fileUTF16BE);
$s = mb_convert_encoding($s, 'UTF-8', "UTF-16BE");
//some operations on $s
file_put_contents($anotherUTF16BEfile, mb_convert_encoding($s, 'UTF-16BE', "UTF-8"));

The second file is in Little Endian (starts with \xFF\FE)!!!

I have to specify LE if I want BE.
file_put_contents($anotherUTF16BEfile, mb_convert_encoding($s, 'UTF-16LE', "UTF-8"));

How come it's reversed?

I forgot to add that I did manage to deal with the UTF-16BE file by reversing everything.

$s = file_get_contents($fileUTF16BE);
$s = mb_convert_encoding($s, 'UTF-8', "UTF-16LE");
//some operations on $s
file_put_contents($anotherUTF16BEfile, mb_convert_encoding($s,
'UTF-16LE', "UTF-8"));

I need to specify "UTF-16LE" in order to be sure I work with "UTF-16BE".

We're also able to reproduce this, with a much smaller test case:

Reproduce code:
---------------
mb_convert_encoding("\xfe\xff\x22\x1e", 'UTF-8', 'UTF-16');


Expected result:
----------------
\xe2\x88\x9e


Actual result:
--------------
\xef\xbb\xbf\xe2\x88\x9e

Alternatively:

Reproduce code:
---------------
bin2hex(mb_convert_encoding("\xfe\xff\x22\x1e", 'UTF-8', 'UTF-16'));


Expected result:
----------------
e2889e


Actual result:
--------------
efbbbfe2889e