Question : How do I validate a dropdownlist in a html form?

The code below is a verisign payment form. Toward the bottom there is  a dropdown list for the user to select a month. I would like this to be a requried field. In which the user cannot proceed to the next page unless a month is selected from the dropdown list. Also I will like for the validation error message to be displayed once the user clicks the submit button after filling out the form.

I don't want to use Asp.net because the website is not hosted on a Microsoft Server. JavaScript or php should be fine.
Thanks!
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Seminar  Registration Form 

              

    Enter information as it corresponds to Credit Card Billing information 

        

        

          

            

               Name
              
                
              
              Company
              
                
             
            

            

               Address
              
                
              
               Phone
              
                
              
            

            

               City
              
                
              
               Fax
              
                
              
            

            

               State
              
                
              
               Email
              
                
              
            

            

              Zip
              
                
              
			  
			     Month
              
                
              
               
            

            

               
            

            

              Please Choose ONE Option:
            

            

               
            

            

              
                
              $25.00  Member Registration
              
                
          $35.00 Non-Member Registration
            

            

               
            

            

               
            

            

              Attendee
              
                
               Lunch 
				Only
               
               
            

            

               
               
               
               
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Answer : How do I validate a dropdownlist in a html form?

Try this code where onsubmit event is added to the form and validation stops the submit if month field is empty
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Seminar  Registration Form 

              

    Enter information as it corresponds to Credit Card Billing information 

        

        

          

            

               Name
              
                
              
              Company
              
                
             
            

            

               Address
              
                
              
               Phone
              
                
              
            

            

               City
              
                
              
               Fax
              
                
              
            

            

               State
              
                
              
               Email
              
                
              
            

            

              Zip
              
                
              
			  
			     Month
              
                
              
               
            

            

               
            

            

              Please Choose ONE Option:
            

            

               
            

            

              
                
              $25.00  Member Registration
              
                
          $35.00 Non-Member Registration
            

            

               
            

            

               
            

            

              Attendee
              
                
               Lunch 
				Only
               
               
            

            

               
               
               
               
Open in New Window Select All
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