In PHP, what are the differences between using $_SESSION variables and $_REQUEST variables, and how should they be properly utilized in a web application workflow?

When developing a web application in PHP, it's important to understand the differences between $_SESSION and $_REQUEST variables. $_SESSION variables are used to store user-specific data across multiple pages, while $_REQUEST variables are used to collect form data submitted via GET or POST methods. It's crucial to properly utilize $_SESSION variables for storing user login information, preferences, or shopping cart data, while $_REQUEST variables should be used to retrieve form data and process user input.

// Example of utilizing $_SESSION and $_REQUEST variables in a web application workflow

// Start the session
session_start();

// Set a session variable
$_SESSION['user_id'] = 123;

// Retrieve form data using $_REQUEST
if ($_SERVER['REQUEST_METHOD'] == 'POST') {
    $username = $_REQUEST['username'];
    $password = $_REQUEST['password'];
    
    // Process the form data
    // Validate user input, authenticate user, etc.
}