In this example, the "is_valid" function is included from a file called "auth.php". The function is then used to check if the user's session token is valid. If the token is not valid, the user is redirected to the login page. If the token is valid, the user is shown the dashboard. It's difficult to determine the specific package or library used here, as different PHP authentication libraries may use different function names or implementations. However, this code example is consistent with common patterns used in PHP authentication libraries, such as checking session tokens against stored user tokens.