WordPress: Custom shortcode

index.php

<?php
/*
Plugin Name: Twitter Shower
Plugin URI: http://vienhoang.com
Description: Display message.
Version: 1.0
Author: Vien Hoang
Author URI: http://vienhoang.com
License: GPL2
*/

// $content is between the shortcode
add_shortcode( 'twitter', function( $atts, $content ) {
	$atts = shortcode_atts(
		array(
			'username' => 'vienhoang',
			'content' => !empty($content) ? $conent : 'Follow me on Twitter!'
		), $atts
	);
	// Extact the atts array and get variables from keys
	extract($atts);

	return "<a href=\"http://twitter.com/{$username}\">{$content}</a>";
} );