Since I have disabled mb support on my servers to get my 
scripts to work I cannot give a concrete example online but 
I can give something here.

My configure string is:
./configure --quiet --with-apxs=/usr/local/apache-1.3.26/
bin/apxs --with-ftp --with-mysql=/usr/local/mysql --with-
pgsql=/usr/local/pgsql --enable-track-vars --enable-
calendar --disable-debug --with-curl=/usr/local/curl --
enable-mbstring --enable-mbstr-enc-trans --with-mcrypt --
with-gd --with-png-dir --with-zlib-dir --with-jpeg-dir

a second server which also had this problem had a configure 
string of:
./configure --quiet --with-apxs=/usr/local/apache-1.3.26/
bin/apxs --with-ftp --with-mysql=/usr/local/mysql --enable-
track-vars --enable-calendar --disable-debug --with-curl=/
usr/local/curl --enable-mbstring --enable-mbstr-enc-trans

Create a form in which some of the form values consist of:

<input type="text" name="myvar[myvar1]">
<input type="text" name="myvar[myvar2]">
<input type="text" name="myvar[myvar3]">

Then submit the form somewhere and echo back 
$myvar['myvar1'], etc or even do a print_r().  You'll find 
(hopefully as i did) that the first four characters of each 
input are chopped off.

In my case the form field names were like this:
submitui[address1]
submitui[address2]
submitui[city]
submitui[state]

I've seen other forms do this successfully (not with the mb 
support compiled in) so I incorporated it into my shopping 
cart program. After adding the two parameters " --enable-
mbstring --enable-mbstr-enc-trans" submissions using this 
form had the first four characters chopped off.

Since this is so weird I don't know if it would have been 
caught before. I would love to see a solution to this since 
I need MB support for a PHP Domain Registrar program we're 
using.