Set variable in succes function to overwrite variable in ajax call

I have the following code:

var offset = 0;

jQuery(document).ready(
    function() {        
        jQuery('ul.productlist').scrollExtend(
            {   
                'target': 'ul.productlist',         
                'url': 'getmore.php?offset=' + offset, 
                onSuccess: function() { 
                    offset = offset + 20;  
                },    
                'newElementClass': 'more'
            } 
        );
    }
);

I am trying to use the new offset the next time the url is called (it is being called every time the page scroll down to get more results i开发者_如何学编程n a dynamic list).

I want to set the offset, so the php page knows which results to serve next. I understand this is not possible, but I can't seem to find a solution.

Any ideas?

'url': function() {
    return 'data.php?offset='+offset;
}

'onSuccess': function() { 
    offset = offset + 20;  
},

passing the url as a function does the trick