Check field content depending on other field entries

I have already an online booking form. The Form works fine now and I would like to add on one more issue, but the code ignores what I want to check. I have 4 fields: "arrival", "departure", "no. of persons" and "comments" to check. Scenario 1: All field mentioned above are empty: Workes fine and message appears: "You have not provided any booking details". Scenario 2: If arrival (date_start) and departure (date_end) is entered, there should be at least an entry either in the field "comment", or in the field "pax". If not, there should be a message: "You have not provided sufficient booking details". INSTEAD: The form is sent, which should NOT be the case ! The code is currently: # all fields are empty : arrival, departure, pax and comments ==> error (works fine) if(empty($data_start) && empty($data_end) && empty($pax)&& empty($comment)){ exit("You have not specified any booking details"); exit; } #If arrival and departure date is entered, there should be at least an entry either in the field "comment", or in the field "pax". if(!isset($data_start) && !isset($data_end) && empty($pax) && empty($comment)){ exit("You have not provided sufficient booking details."); exit; } The form is attached and can be tested at [login to view URL] Can someone please check and tell me what's wrong with the code ?