Erro php Warning: Cannot modify header information

Boa noite, estou com o seguinte erro no php:

Estou usando o sistema de login desse site:

http://blog.ultimatephp.com.br/sistema-de-login-php/

Warning: Cannot modify header information - headers already sent by (output started at /home/vuzz/public_html/btc/admin/index.php:89) in /home/vuzz/public_html/btc/admin/conecta/check.php on line 7