What common issue arises when using a PHP login system that restricts multiple users from logging in simultaneously?

The common issue that arises when using a PHP login system that restricts multiple users from logging in simultaneously is that users may encounter errors or be unable to login if another user is already logged in with the same credentials. This can be solved by implementing a session management system that tracks active user sessions and prevents multiple logins with the same credentials.

// Start session
session_start();

// Check if user is already logged in
if(isset($_SESSION['user_id'])) {
    // Redirect user to dashboard or display error message
    header("Location: dashboard.php");
    exit();
}

// Validate user credentials and log them in
// Add code for validating user credentials here

// Set session variables
$_SESSION['user_id'] = $user_id;
$_SESSION['username'] = $username;

// Redirect user to dashboard
header("Location: dashboard.php");
exit();