Problemas con sistema de usuarios de php

[Cristo214]

Perdón que siga jodiendo, pero ahora me dice lo siguiente:


Warning: Cannot modify header information - headers already sent by (output started at D:\xampp\htdocs\comer\users\comprueba.php:3) in D:\xampp\htdocs\comer\users\comprueba.php on line 23

Warning: Unknown: Your script possibly relies on a session side-effect which existed until PHP 4.2.3. Please be advised that the session extension does not consider global variables as a source of data, unless register_globals is enabled. You can disable this functionality and this warning by setting session.bug_compat_42 or session.bug_compat_warn to off, respectively. in Unknown on line 0

Que puedo hacer????