Description:
------------
When trying to operate with binary data using substr and mb_internal_encoding is set to UTF-8 (but no function overloading is set) substr works wrong - wrong number of bytes is returned after function call

P.S. emptyfile.xls used in example is an empty MS Excel 2003 file. It can be downloaded at http://an-best.ru/empty_file.xls (13/5 Kbytes)

P.P.S. IDENTIFIER_OLE constant is taken from Spreadsheet_excel_reader class.

Reproduce code:
---------------
<?php

echo "function overload = ".ini_get('mbstring.func_overload')."<br />\n";

// Uncomment this for demonstration of wrong behaviour
//mb_internal_encoding('UTF-8');


echo "MB_INTERNAL_ENCODING =".mb_internal_encoding()."<br />\n";

define('IDENTIFIER_OLE',
pack("CCCCCCCC",0xd0,0xcf,0x11,0xe0,0xa1,0xb1,0x1a,0xe1));

$data = file_get_contents($_SERVER['DOCUMENT_ROOT'].'/substr_bug/emptyfile.xls');

echo "Data length = ".strlen($data)."<br />\n";
echo "First 8 symbols  ==>".var_export(substr($data,0,8),1)."<== <br
/>\n";
echo "Compare result (substr(\$data,0,8)==IDENTIFIER_OLE) -
".var_export(substr($data,0,8)==IDENTIFIER_OLE,1)."<br />\n";
echo "Substring length (substr(\$data,0,8)) -
".strlen(substr($data,0,8))."<br />\n";

?>

Expected result:
----------------
function overload = 0
MB_INTERNAL_ENCODING =ISO-8859-1
Data length = 13824
First 8 symbols ==>'&#1087;&#1086;&#1070;&#9553;&#9568;&#1040;'<==
Compare result (substr($data,0,8)==IDENTIFIER_OLE) - true
Substring length (substr($data,0,8)) - 8


Actual result:
--------------
// This result can be seen if mb_internal_encoding is set to UTF-8

function overload = 0
MB_INTERNAL_ENCODING =UTF-8
Data length = 13824
First 8 symbols ==>'&#1087;&#1086;&#1070;&#9553;&#9568;&#1040;' . "\0"
. '' . "\0" . '' . "\0" . '' . "\0" . '' . "\0" . ''<==
Compare result (substr($data,0,8)==IDENTIFIER_OLE) - false
Substring length (substr($data,0,8)) - 13