Simple Session-Based Flash Message

Set the Session just in case

 //Ensure that a session exists (just in case)
if( !session_id() )
{
    session_start();
}

The rest is as easy as it gets!

The Function.

/**
 * Function to create and display error and success messages
 * @access public
 * @param string session name
 * @param string message
 * @param string display class
 * @return string message
 */
function flash( $name = '', $message = '', $class = 'success fadeout-message' )
{
    //We can only do something if the name isn't empty
    if( !empty( $name ) )
    {
        //No message, create it
        if( !empty( $message ) && empty( $_SESSION[$name] ) )
        {
            if( !empty( $_SESSION[$name] ) )
            {
                unset( $_SESSION[$name] );
            }
            if( !empty( $_SESSION[$name.'_class'] ) )
            {
                unset( $_SESSION[$name.'_class'] );
            }

            $_SESSION[$name] = $message;
            $_SESSION[$name.'_class'] = $class;
        }
        //Message exists, display it
        elseif( !empty( $_SESSION[$name] ) && empty( $message ) )
        {
            $class = !empty( $_SESSION[$name.'_class'] ) ? $_SESSION[$name.'_class'] : 'success';
            echo '<div class="'.$class.'" id="msg-flash">'.$_SESSION[$name].'</div>';
            unset($_SESSION[$name]);
            unset($_SESSION[$name.'_class']);
        }
    }
}

Setting Messages

//Set the first flash message with default class
flash( 'example_message', 'This content will show up on example2.php' );

//Set the second flash with an error class
flash( 'example_class', 'This content will show up on example2.php with the error class', 'error' );

Displaying the Messages

<?php flash( 'example_message' ); ?>
<?php flash( 'example_class' ); ?>