how to create a secure php login system, allowing for "keep me logged in" functionality?

Question

I use a simple login system based on SESSION vars. Once the user logs in, a session var is set that tells my script the user is to be accepted in. I don't use any custom clientside cookie var.

I would like to offer the option on the login screen that says "keep me loggued in the whole day". How does one do that in a secure way?

Answer 1

First: Configure the session.cookie_lifetime directive, either in php.ini, configuration files, or via session_set_cookie_params().

Next, store the username and the hash value of the password in the session, and validate that login on every page. As long as it's still valid, they get to stay logged in.

The session cookie's natural expiration should generally keep things tidy, as you won't have anyone getting logged out in the middle of their session (if the stars aligned for it, of course) if they keep it active. Failing that, though, I'd consider eCartoth's solution a close second, as you could just add a second line to the if statement:

if (my_validate_user_function($_SESSION['username'],$_SESSION['passhash']) 
    && $_SESSION['deathstamp'] > time()
) {
    // user is logged in again, oh boy!
}
else {
    // send in the death robots
    header('Location: /login.php',true,302);
}

EDIT: One thing you might want to consider is session fixation and/or session hijacking. In order to prevent that, I'd recommend one (or both) of two solutions:

1. store the user's IP address in the session
2. use session_regenerate_id() after every successful login attempt.