How can the inclusion of login scripts affect the retention and accessibility of session variables in PHP applications?

Including login scripts can affect the retention and accessibility of session variables in PHP applications if the session is not properly started and managed within the login script. To ensure that session variables are retained and accessible throughout the application, the session_start() function should be called at the beginning of the login script and any other scripts that require access to session variables.

<?php
// login script
session_start();

// perform login logic

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

// redirect to dashboard or home page
header("Location: dashboard.php");
exit;
?>