set_cookie problem =/ ~ beginner

[eluser]cehennem[/eluser]
I am trying to set a cookie like this;

set_cookie('username', $this->input->post('user'), 10); // 10 secs for test

it doesn't work for me.

But when I try higher values such as 5000, 30000 etc, it works nice (not sure when it's going to be expired)

I haven't post the full code coz of that interesting issue. It doesn't accept my small number of seconds.

Can somebody tell me whats wrong?