How can the use of session variables in PHP impact the performance and security of a web application?

Using session variables in PHP can impact performance if they are not managed efficiently. Storing large amounts of data in session variables can lead to increased server load and slower response times. Additionally, storing sensitive information in session variables without proper security measures can make the application vulnerable to attacks like session hijacking. To improve performance and security, it's important to only store necessary data in session variables and limit the size of the stored data. Additionally, sensitive information should be encrypted before storing it in session variables.

// Start the session
session_start();

// Store a limited amount of necessary data in session variables
$_SESSION['user_id'] = $user_id;
$_SESSION['username'] = $username;

// Encrypt sensitive information before storing it in session variables
$encrypted_data = encrypt($sensitive_data);
$_SESSION['encrypted_data'] = $encrypted_data;