Wordpress Mailchimp Subscriber Shortcode

<? 
//=========================================================== SUBSCRIBER SHORTCODE
function subscriber_shortcode($atts, $content = null ) {
	extract(shortcode_atts(array(
		'page' => '',
		'headline' => 'Subscribe for free updates',
		'button' => 'Subscribe',
		'quote' => '',
		'class' => 'box_main',
		'form_class' => 'centered_form',
		'btn_class' => '',
		'giveaway' => '',
		'success' => 'Alright! You&rsquo;re on the list. Excited to have you on board.',
	), $atts));
	
	if($class != 'none') { $return_string = '<div class="'.$class.'">'; }	
		$return_string .= '<form method="post" class="'.$form_class.' fz_subscribe_form" action="" id="">';
			if($headline != 'none') { $return_string .= '<h2>'.$headline.'</h2>'; }	
			if($content != '') { $return_string .= '<p>'.$content.'</p>'; }			
			$return_string .= '
				<p class="error text_alert" style="display:none"></p>
				<input class="text" type="email" name="email" required="required" placeholder="Enter your email" />
				<input type="hidden" name="formloc" value="'.$page.'" />
			';
			if(!empty($giveaway)) { $return_string .= '<input type="hidden" name="giveaway" value="'.$giveaway.'" />'; }			
			$return_string .= '	
				<input class="submit no_margin '.$btn_class.'" type="submit" name="submit" value="'.$button.'" onClick="_gaq.push([\'_trackEvent\', \'Email\', \'Subscribe\', \''.$page.'\']);" />
				<img src="http://fizzle.co/images/loading.gif" id="loadingGif" style="display:none;"/>
				<p class="success text_alert" style="display:none">'.$success.'</p>
			';		
		$return_string .= '</form>';		
		if($quote != '') {
			$return_string .= '
				<br/>
				<p class="center quiet"><em>'.$quote.'</em></p>
			';
		}		
	if($class != 'none') { $return_string .= '</div>';	 }
	return $return_string;
}
add_shortcode('subscribe', 'subscriber_shortcode');



//=========================================================== USAGE: 
// [subscribe headline="Get our insights stream" page="about_top" giveaway="AUDGUIDE" success="Awesome! We&rsquo;ll send you the guide shortly." quote="&ldquo;I have been dreaming of this blog for quite some time, so I am beyond thrilled that I am making it a reality.  Fizzle has been a tremendous help in giving me the knowledge, confidence and courage to get this off the ground. Thank you!!!&rsquo; ~ Denise R."]Enter your email and we&rsquo;ll send you a weekly email to help you grow your online business. We&rsquo;ll also send you our guide to defining your audience, the FIRST step in knowing how to find, delight and grow your audience.[/subscribe]