function showads() {  
 return '<div id="adsense"><script type="text/javascript"><!--  
 google_ad_client = "pub-XXXXXXXXXXXXXX";  
 google_ad_slot = "4668915978";  
 google_ad_width = 468;  
 google_ad_height = 60;  
 //-->  
 </script>  
 
 <script type="text/javascript"  src="http://pagead2.googlesyndication.com/pagead/show_ads.js">  
 </script></div>';  
 }  
 
 add_shortcode('adsense', 'showads');