无极分类2

 <?php
 $arr = [
['id'=>1, 'parent_id'=>0, 'name'=>'aaa'],
['id'=>2, 'parent_id'=>0, 'name'=>'bbb'],
['id'=>3, 'parent_id'=>0, 'name'=>'ccc'],
['id'=>4, 'parent_id'=>2, 'name'=>'ddd'],
['id'=>5, 'parent_id'=>2, 'name'=>'eee'],
['id'=>6, 'parent_id'=>5, 'name'=>'fff'],
['id'=>7, 'parent_id'=>2, 'name'=>'ggg'],
['id'=>8, 'parent_id'=>3, 'name'=>'hhh'],
];



//echo '<select>'.make_tree($arr).'</select>';
//$tree = make_tree($arr);
$tree = make_tree($arr);
$tree = select($tree);
echo '<select>'.$tree.'</select>';

function make_tree($arr, $parent_id=0, $deep = 0){
    $deep += 2;
    $new_arr = array();
    foreach($arr as $k=>$v){
        if($v['parent_id'] == $parent_id){
            //$v->name = str_repeat(' ', $deep) . $v->name;
            $v['children'] = make_tree($arr, $v['id'], $deep);
            $new_arr[]   = $v;
        }
    }

    return $new_arr;
}
 
function select($tree, $deep = 0) {
    $options = ''; $deep+=2;
    foreach ($tree as $row) {
        $row['name'] = str_repeat(' ', $deep) . $row['name'];
        $options .= "<option>{$row['name']}</option>";
        if ($row['children']) {
            $options .= select($row['children'], $deep);
        }
    }
    return $options;
}