What are the implications of allowing users to be logged in with the same credentials on multiple devices simultaneously in a PHP login system?

Allowing users to be logged in with the same credentials on multiple devices simultaneously can pose a security risk as it increases the chances of unauthorized access to the account. To prevent this, you can implement a session management system that tracks active sessions and allows only one session per user at a time. This can be achieved by storing a unique session identifier in the database for each user and checking it during login to ensure only one session is active.

// Check if user is already logged in on another device
function checkActiveSession($user_id) {
    // Query database to check if user has an active session
    // Return true if active session found, false otherwise
}

// Log in user and create session
function login($username, $password) {
    // Verify username and password
    // Check if user already has an active session using checkActiveSession function
    // If active session found, invalidate it
    // Create new session for user
}