I have a table of members from a database. I want to filter the results on the same page without reloading using the following form

HTML Code:
<select name="club" onchange="filter(this.value)">
				<option value="">Select A Club</option>
				<option value="1">1</option>
				<option value="2">2</option>
			</select>
The ajax code is

HTML Code:
function filter(status){
    var url = "index.php";
    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.open("POST", url, true);
    xmlhttp.setRequestHeader("Content-type","application/x-www-form-urlencoded");
    xmlhttp.send("status="+status);
}
the php on the page doesnt catch the post
PHP Code:
$sql "SELECT * FROM members ";
                if(
$_POST){
                    
$club $_POST['status'];
                    
$sql .= "WHERE club = $club ";
                }
            
$sql .= "ORDER BY surName DESC"
Any help please