What potential issues can arise when using session flags to track user activity in PHP?

Using session flags to track user activity in PHP can lead to potential issues such as inconsistent tracking if the flag is not properly updated or reset. To solve this, it is important to update the flag whenever user activity occurs and reset it when necessary, such as when the user logs out or the session expires.

// Update session flag when user activity occurs
$_SESSION['user_active'] = true;

// Reset session flag when user logs out or session expires
unset($_SESSION['user_active']);