submit form problem

Well, here's the problem I faced. I've a script accepting four different forms depending on the user choice. Some of the forms have two submit buttons forcking actions accordingly to the pressed button. The problem is: $HTTP_POST_VARS["submit"] is not set if the user press ENTER instead of clicking the button and the scripts cannot return a response for it is not clear which button has been pressed. Hidden field cannot works with two submit buttons for I'll have two values, but again cannot cach the user choice in case of ENTER press. Any help? Thank you Ivo