php error message

I am new to working with php and I am having trouble trying to figure out this following "error" message that is displayed when I access a php file. Can you tell me what this error message means (in plain English) and what steps I might take to correct it? "Warning: Cannot modify header information - headers already sent by (output started at /usr/home/ecmartin/public_html/ethics06/calendarlogin.php:8) in /usr/home/ecmartin/public_html/ethics06/sas.php on line 34" The page that displays this error has a small php log-in script on in. (Depending on where I place the script in the page code, the error message moves to a different location on the page). The page originally was an html page that contained typical stylesheet data, javascript scripts (for headers and menus) and a flash-based menu. ALL of these features still work when the page is displayed (as well as the php log-in script), however, I cannot get rid of the error message. Thanks