How to create a dropdown multilevel tree view?

Question

PHP

The essence of the question is that we need a drop-down multi-level tree-like list that receives data from the database, there is already a code, with the only exception that the expansion is not configured, here is the result screen:

here is the code

<?
include_once("bd.php");
header("Content-Type: text/html; charset=UTF-8");
//Выбираем данные из БД
$result=mysql_query("SELECT * FROM  table1");
//Если в базе данных есть записи, формируем массив
if   (mysql_num_rows($result) > 0){
    $cats = array();
//В цикле формируем массив разделов, ключом будет id родительской категории, а также массив разделов, ключом будет id категории
    while($cat =  mysql_fetch_assoc($result)){
        $cats_ID[$cat['uid']][] = $cat;
        $cats[$cat['pid']][$cat['uid']] =  $cat;
    }
}
function build_tree($cats,$parent_id,$only_parent = false){
    if(is_array($cats) and isset($cats[$parent_id])){
        $tree = '<ul>';
        if($only_parent==false){
            foreach($cats[$parent_id] as $cat){
                $tree .= '<li>'.$cat['name'].' #'.$cat['uid'];
                $tree .=  build_tree($cats,$cat['uid']);
                $tree .= '</li>';
            }
        }elseif(is_numeric($only_parent)){
            $cat = $cats[$parent_id][$only_parent];
            $tree .= '<li>'.$cat['name'].' #'.$cat['uid'];
            $tree .=  build_tree($cats,$cat['uid']);
            $tree .= '</li>';
        }
        $tree .= '</ul>';
    }
    else return null;
    return $tree;
}
echo build_tree($cats,0);
?>

Answer 1

Here is a simple Jquery example - https://codepen.io/anikey99/pen/GJLdrZ