header,session stuff works on live fails on dev...?

Hi all, I am still failing to fix this problem with headers. I receive the following error: Warning: Cannot add header information - headers already sent by (output started at c:\apache\apache\htdocs\client-secure.php:11) in c:\apache\apache\htdocs\client-secure.php on line 18 Which would indicate that the line 11 is sending output to the browser. Line 11 is: $uid=$HTTP_SESSION_VARS["userid"]; How can this be sending an output? I also get this: Warning: Undefined index: userid in c:\apache\apache\htdocs\client-secure.php on line 11 Which would indicate that it cant find the session info. If it cant find it does it write it? This is not an isolated case. This problem is happening in a few areas of the system yet on the live server everything works a dream!!?? Anyone any ideas? Cheers Nic