Some browsers only transmit the name/value pair for the submit form element when the submit button is clicked. If the form is submitted with the 'Enter' key, then the name/value pair for the submit element is not sent (method POST or GET). All the other form element name/value pairs appear to be sent, including hidden elements.
Only some browsers act this way. I believe IE 7.0 is one.
Therefore you cannot use the name/value pair for the submit button as a reliable test for the form being submitted. Hidden elements do show up.
I have experienced this problem with PHP5, but not with PHP4. Cannot explain that one.