both of you probably have the "register_globals" option set to "OFF"
if you just upgraded to PHP 4.2.1, this is probably the problem - if your script tries to access a variable via the variable name itself instead of its global array:
Ex:
url is: http://.........../page.php?var1=foo&var2=bar
if register_globals is off, you can't just refer to var1 as $var1, you have to use the global request method array $HTTP_POST_VARS["var1"] or $HTTP_GET_VARS["var1"]