session_start(); // code to process user input if (success) { $_SESSION['flash_message'] = "Your action was successful!"; } else { $_SESSION['flash_message'] = "There was an error with your action."; } // redirect the user to another page header("Location: /some-other-page.php"); exit();In this example, a flash message is created based on the outcome of some process (denoted here by the "success" variable). The message is stored in a session variable called "flash_message." Later, the user is redirected to another page, where the message can be displayed. The set_flash() function is typically included as part of a larger package or library, designed to streamline the process of storing and retrieving session data in PHP. Some popular packages that include set_flash() functionality include Laravel, Symfony, and CodeIgniter.