how to clear up text field on click?

How do I make submit button not function if the text field is empty?

• I have a search bar at the top of my website and it works when you type stuff in it and click "search" but when the text field is empty and you click the search button, it brings you to my 404 error page. I would like the search button to not function if nothing is in the text field. Here is an example search I found: http://www.t-mobile.com/

• Answer:

  Use PHP to redirect to a different page if there's nothing in the field maybe? Or use javascript to make the button not function if the bar is empty?