confusing problem w/sessions

Dear sirs; i have a simple snippet of code : <? session_start(); $a = 10; session_register("a"); ?> i am trying to understand how do session work. Accessing at this document directly from the server (it is a SuSE 7.2 PE - running PHP 4.0.4 and apache 1.3.9) i find out the following problem: Warning: cannot send session cookie - headers already sent (output started at ....) and Warning: cannot send session cache limiter - headers already sent (output started at ....) The matter is that if i access this variables from another page, they seem to be empty... How can i solve this little confusing problem? Thanks in advance Gabriele