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

Using session variables in PHP can impact performance and scalability of a web application because it requires server-side storage and retrieval of session data for each user, which can consume server resources and slow down the application. To improve performance and scalability, consider using alternative methods such as storing session data in a database or using caching mechanisms.

// Example of storing session data in a database for improved performance and scalability
session_set_save_handler(
    new CustomSessionHandler(), // Custom session handler class
    true // Register the custom session handler
);
session_start();