How to pass php variable value returned by javascript to a php variable?

HTML (When this DIV is clicked a call to searchfunction made and its id is passed)

<div class="me" id="<?php echo $row['main_cat_id'];?>" onClick="searchfunction('<?php echo $row['main_cat_id'];?>');">
    <?php echo $row[ 'main_cat_name'];?>
</div>

Below is the search function; it passes the ID value of the clicked DIV to the pagegetparam.php.

 function searchfunction(param) {
    //alert(param);
    var xmlhttp = getXmlHttp();
    xmlhttp.onreadystatechange = function () {
        if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
            if (this.responseText !== null) {
                var ajaxElm = document.getElementById('getparam');
                ajaxElm.innerHTML = this.responseText; // append in front
                //jQuery(ajaxElm).append(this.responseText);
            }
        }
    }
    xmlhttp.open("GET", "getparam.php?param=" + param, true);
    xmlhttp.send();
}

And getparam.php only passes back the ID to the same page it got it from. I've resorted to this method, because I believe there's no way to directly pass the id of the clicked div which was captured by JavaScript and assign it to a php variable.

<?php
$param=$_GET['param'];
echo $param;
?>

Now, I need the value of clicked div id to run this query below:

echo $param='<span id="getparam"></span>';//it does echo the id in numeric like, 1 or 2 .But it doesn't help to execute the query though.

 $sql= "SELECT * FROM categories,main_category WHERE categories.main_cat_id=main_category.main_cat_id AND categories.main_cat_id='$param'";

How can I get the clicked DIV id passed to the php variable $param? Is there any other way or can anything be improved here?