With

 php_value mbstring.internal_encoding "UTF-8"
 php_value mbstring.func_overload 6

in my php.ini, the overloaded strrpos() gives a wrong result when parsing an UTF-8 string.
For example,

 $s=iconv("ISO-8859-1","UTF-8","?.jpg");
 $x=strrpos($s,".");

assigns the value 2 to $x, instead of the correct one (1).

My PHP was configured with
./configure  --with-config-file-path=/etc/httpd/conf --with-apxs --with-pgsql 
--with-iconv --enable-mbstring --enable-mbregex

A patch which fixes this bug can be found at
http://village.flashnet.it/users/mi1802/php-4.3.1-mbstring.diff

Cheers,
 David