Source code for: http://baylorrae.com/add-flash-messages-to-your-site/

displaying.php

<?php include 'flash.php'; ?>
...
<?php foreach( Flash::$messages as $id => $msg ) : ?>
  <div class="flash_<?php echo $id ?>"><?php echo $msg ?></div>
<?php endforeach; ?>

flash.class.php

<?php
// make sure sessions work on the page
session_start();

class Flash {

  // where all messages are stored
  public static $messages = array();

  /*
   * A generic function to store flash messages
   *
   * Flash::add('notice', 'a message to display');
   *
   * @param string $name the name/id of the flash
   * @param string $message the message to display
   */
  public static function add($name, $message) {
    $_SESSION['flash_messages'][$name] = $message;
  }

  /*
   * A shortcut to Flash::add()
   *
   * Flash::notice('a message to display');
   */
  public static function __callStatic($fn, $args) {
    call_user_func_array(array('Flash', 'add'), array($fn, $args[0]));
  }
}

// if $_SESSION['flash_messages'] isset
// then save them to our class
if( isset($_SESSION['flash_messages']) ) {
  self::$messages = $_SESSION['flash_messages'];
}

// reset the session's value
$_SESSION['flash_messages'] = array();

usage.php

<?php

include 'flash.php';

// using Flash::add directly
Flash::add('notice', 'Awesome job dude!');

// using the alias
Flash::alert('lol, you can\'t do that!');
// calls: Flash::add('alert', 'lol, you can\'t do that!');