Why is the behavior between Apache2 and Tomcat so different?

Hi, I have the same piece of PHP script running on either Apache2 or Tomcat. On Tomcat, I have the folllowing errors: 1- Cannot modify header information - headers already sent .... OR 2- Undefined index: error in ... For one, I think this is because I am using the header PHP function. For two, I think this is because there is from time to time a http request parameter ("error") which not always defined. Could someone explain me why this is the case? And how I can change this behavior. Thanks. David Joffrin www.etradinginnovations.co.uk