<?php
/*
Plugin Name: Twitter Shower
Plugin URI: http://www.scoldy.com
Discription: Simple shortcode
Version: 1.0
Author: Ronnie
Author URI: http://www.scoldy.com
*/

add_shortcode('twitter', function($atts,$content) {
	$atts = shortcode_atts(
	 array(
	 'username' => 'VeronicaDjeneva',
	 'content' => !empty($content)? $content : 'Follow me on Twitter!'
	 ),$atts
	 );
	 extract($atts);
	 
	 return "<a href='http://twitter.com/$username'>$content</a>";
});

?>

Wordperss Follow with variable 
In double quotes you have ability to embed variable
post content: [twitter username='VeronicaDjeneva']Follow me Fools[/twitter]

Wordperss Follow with variable

PHP