Tree

function tree($parent){
    global $con;
    $sql = "select * from category where parentid =$parent";
    $result =  $con->query($sql);

    while ($row = $result->fetch_assoc()) {
            echo $row['childid']." ".$row['childname']."<br>";
            tree($row['childid']);

    }
}

$tree(select all id where parentid is null)