[stonyarc]

I found it.

The error were caused by a configuration in the php.ini

output_buffering should be on instead of off otherwise the cookie isn't set.

It's impossible to call a PHP function directly from javascript but there is a nice workaround for this.

Here is the code to fix it (this one works with vbulletin, if you build one with an image it doesn't work).

<script language="Javascript">
    var myUrl="./function.php?sw="+screen.width+"&sc="+screen.colorDepth+"&referer="+document.referrer+"&page="+location.href;
	var iframe1=document.createElement("iframe");
	iframe1.src=myUrl;
	document.body.appendChild(iframe1);
	iframe1.removeNode(true); //hides the frame from the user
</script>

Enjoy