Changing the contents of a div using a dropdown, javascript and PHP

I'm a noob but will gladly take some stick for it if you help me find the answer ;)

I have a list of categories in an array $cat_array. I want to build a select drop-down so that when I select one of the options, a different variable is passed to a function.

This is what I have so far: In the main file (index.php), I call a function:

makechoice($cat_array);

This refers to this function which is in an included file:

function makechoice($cat_array) {   
$dbz = new db();
$sim = new simple();
echo '<div class="choose-section">';
    echo '<select class="selbox" onchange="categoryAjaxData(\'facebook\',\'/includes/fancount.php\',this.value);">';
        foreach($cat_array as $cat) {   
            echo('<option value="'.$cat[0].'">'.$cat[1].'</option>');   
        }
    echo '</select>';
    echo 'Choose a section';
    echo '</div>';
    echo '<div id="facebook"><div class="facebook-midcut">',showfans($djnames, $djids, $djurl),'</div><div class="l-botcut"></div></div>';
}

In includes/fancount.php, I have basically have lots of different variables that I want to pass through the showfans function.

e.g. $barnames, $barids, $barurl & $restaurantname, $restaurantids, $restauranturl

The showfans function uses these variables to display some data about them.

This is my javascript code:

function categoryAjaxData(div,str,value)
{
    var url = str+'?catid='+value;
    ajaxData(div,url);
}

function ajaxData(div,str)
    {
        xmlHttp=GetXmlHttpObject();
        document.getElementById(div).innerHTML='<center><img src="images/loader.gif"></center>';        
        if(xmlHttp==null)
        {
            alert("Browser does not support HTTP Request")
            return
        }
            var url = str;
            xmlHttp.onreadystatechange = function(){ DescriptionstateChanged(div); };
            xmlHttp.open("GET",url,true);
            xmlHttp.send(null);
    }

function DescriptionstateChanged(div)
{
    if(xmlHttp.readyState==4 || xmlHttp.readyState=="complete")
    {
        document.getElementById(div).innerHTML=xmlHttp.responseText;
    }
} 

AT THE MOMENT: The content in the current facebook div reloads, I can see the loader - but I'm stuck with how to link the option selected in the select dropdown to changing the variables in the showfans() function.

E.g. I want "Bar" in the dropdown to show:

echo '<div id="facebook"><div class="facebook-midcut">',showfans($barnames, $barids, $barurl),'</div><div class="l-botcut"></div></div>';

I want "Restaurant" in the dropdown to show:

echo '<div id="facebook"><div class="facebook-midcut">',showfans($restaurantnames, $restaurantids, $restauranturl),'</div><div class="l-botcut"></div></div>';

I hope that helps! Thanks in advance :)

UPDATE:

With regards to fancount.php, I have been looking at various different options - but I can't seem to get any of them to work!

I want to pass the value from the dropdown into a logic so that depending on the varlue chosen, certain variables are used by showfans() and the output code looks similar to this for the various combination of variables:

echo '',showfans($restaurantnames, $restaurantids, $restauranturl),'';