erro ao logar PHP

Após o log in no meu site ele aparece com os seguintes erros :

Warning: session_regenerate_id() [function.session-regenerate-id]: Cannot regenerate session id - headers already sent in /home/jose/public_html/cabecalho.php on line 74

Warning: Cannot modify header information - headers already sent by (output started at /home/jose/public_html/index.php:8) in /home/jose/public_html/cabecalho.php on line 83

me ajudem por favor !