I just tested again this, and could finally reproduce this.
(had an old cgi binary which read wrong php.ini, stupid me..)

Summary of conditions:

- only happens with the regular posts (ie. when rfc1867.c is used, it works fine, or GET method)

- register_globals has to be 'On'

- The posted value has to be longer than one (1) character

- Only the global variable gets mangled, $_POST[] array has the correct value

making critical (should really be fixed before 4.3.2 goes gold..)

How are you triggering the cgi?  ie. what does your Apache config look like?

It doesn't matter. I tried two different ways, both failed.
For example (otherwise default httpd.conf):

    AddType application/x-httpd-php .php
    ScriptAlias /php/ "/www/apache/cgi-bin/"
    Action application/x-httpd-php /php/php-cgi

That means a command-line test case in tests/basic should fail as well.

Related to bug #22612

This problem is a rather interesting one, it appears that in CGI the entire POST data makes into the enviroment. You can see this by adding a printf() to _php_import_environment_variables() function. The last element will contain the entire POST string. If that string contains a '=', then data before the '=' becomes the key (variable name) and the rest the variable content. Meaning that by the time the POST variables are registered $suchstr already has a value.
One way to fix this is to prevent duplicate variable registration by breaking on the 1st invalid entry from the enviroment inside _php_import_environment_variables(), which happens to be the script name, which is followed by either the POST data or random garbage.
Here is the relavent patch: http://bb.prohost.org/22773.txt

This bug has been fixed in CVS.

In case this was a PHP problem, 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/.
 
In case this was a documentation problem, the fix will show up soon at
http://www.php.net/manual/.

In case this was a PHP.net website problem, the change will show
up on the PHP.net site and on the mirror sites in short time.
 
Thank you for the report, and for helping us make PHP better.

It looks like I'm getting the same error, but in slightly different conditions.

I'm on PHP 4.2.2 and have register globals off.  I get exactly the same error:
When I submit the value "bar", and I print_r  the  _POST, I expect to get the output:
  Array ( [foo] => bar ) 
but instead I get:
  Array ( [foo] => barfoo=bar ) 

I just thought it was important to note that it also happens with these conditions:
- register_globals can be 'Off'

- The $_POST[] array is mangled too

- $_GET is unaffected

I am seeing the exact same problem as jcarver described.  I don't see this problem when I am using Mozilla 1.1 but I see this problem with both IE 5 and 6.