<?php
$file = $_SERVER['DOCUMENT_ROOT'].'/import.xml';
$xml = simplexml_load_file($file); 

$json = json_encode($xml);
$array = json_decode($json,TRUE);

$group = [];
function get_group($data, &$group, $parent = ''){
    foreach($data as $key=>$value){
        if(isset($data['Ид'])){
                $group[$data['Ид']] = [
                    'id' => $data['Ид'],
                    'parent_id' =>  $parent,
                    'name' => $data['Наименование']
                ];

            if($key == 'Группы' && isset($data[$key]['Группа'])){
                get_group($data[$key]['Группа'], $group, $data['Ид']);
            }
        }else{
            if(is_array($data)){
                foreach($data as $k => $v){
                    get_group($data[$k], $group, $parent);
                }
                
            }
        } 
    }
}

get_group($array['Классификатор'], $group);
echo '<pre>';
print_r($group);
echo '</pre>';
exit;