Load gallery shortcode with ajax

Heres an easy for you all!
I want to use next gen gallery using ajax to load the shortcode of my page. I use some links inside the page so than when i click them to load the new next gen shortcode without reloading the page.

Things i have done:
1. I have added the following shortcode in the body of my webpage:

<div id="gallery"><?php echo do_shortcode("[slideshow id=1 w=800 h=100]"); ?></div>

2.Inside a page i have added the following links:

<a href="#" onClick="show(1);" id="gallery">Click to show Gallery 1</a>
<a href="#" onClick="show(2);" id="gallery">Click to show Gallery 2</a>

3.The ajaxcode:

<script type="text/javascript">
// var site = $site;
function show(num){
	var gallery_id = num;
	var xmlhttp;
	if (window.XMLHttpRequest)
	  {// code for IE7+, Firefox, Chrome, Opera, Safari
	  xmlhttp=new XMLHttpRequest();
	  }
	else
	  {// code for IE6, IE5
	  xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
	  }
	xmlhttp.onreadystatechange=function()
	  {
	  if (xmlhttp.readyState==4 && xmlhttp.status==200)
	    {
	    document.getElementById("gallery").innerHTML=xmlhttp.responseText;
	    }
	  }
	xmlhttp.open("POST","wp-content/themes/twentytwelve/ajax_info.php",true);
	xmlhttp.setRequestHeader("Content-type","application/x-www-form-urlencoded");
	xmlhttp.send("gallery=" + gallery_id);
	gallery.innerHTML="requesting...";
}
</script>

4.The ajax info file contains the following:

<?php echo do_shortcode("[slideshow id=".$_POST['gallery']." w=800 h=100]"); ?>

But i get a php error.
Any solution/ideas for fixing that?? Or any other approach??

Thank you in advance!
Kostas

http://wordpress.org/extend/plugins/nextgen-gallery/