Is there a Drupal function to check if the user is logged in?

Yes, and the function to use depends on the version of Drupal that you're using with your website.

Drupal 6 and 7

The function to use is called user_is_logged_in, and it can determine if a given user is logged in. For complete documentation on the function, click the appropriate link from the following, based on your Drupal version:

• Drupal 7 user_is_logged_in function information from drupal.org
• Drupal 6 user_is_logged_in function information from drupal.org

The function is a boolean that returns TRUE if the user is logged in, and FALSE if the user is anonymous.

Drupal 8

There are a couple of methods that you can use in Drupal 8 to make this check:

• Use the isAuthenticated() method on the user. For example, use the following code to determine the state of the current user:

  $logged_in = \Drupal::currentUser()->isAuthenticated();

• Use isAnonymous() to determine if the user is anonymous:

  \Drupal::currentUser()->isAnonymous();