Difference between equal $_SERVER variables?

Hi list, Just out of curiosity I would like to know if anyone can tell me the difference between some $_SERVER variables that generate the same output (in my test-file at least). For instance, what is the difference between $_SERVER['HTTP_HOST'] and $_SERVER['SERVER_NAME']? Can they generate different output under some circumstances? They're always the same with me. Any recommendations on which to use? I know that $_SERVER['REQUEST_URI'] can differ from 'SCRIPT_NAME' and 'PHP_SELF' (I use 'REQUEST_URI' to see what people were looking for when they hit a 404) but can $_SERVER['SCRIPT_NAME'] and $_SERVER['PHP_SELF'] be different? TIA, -- Ivo Fokkema PHP & MySQL programmer Leiden University Medical Centre Netherlands