MYSQL親子同じテーブル。多次元配列としての親内のPHPネストの子

参照、順序は重要ではないという利点があります（子ノードは親ノードの前に来ることができます）：

 $tree = array('NULL' => array('children' => array()));
 foreach($array as $item){
    if(isset($tree[$item['id']])){
       $tree[$item['id']] = array_merge($tree[$item['id']],$item);
    } else {
       $tree[$item['id']] = $item;
    }

    $parentid = is_null($item['id_parent']) ? 'NULL' : $item['id_parent'];
    if(!isset($tree[$parentid])) $tree[$parentid] = array('children' => array());
    //this & is where the magic happens: any alteration to $tree[$item['id']
    //  will reflect in the item $tree[$parentid]['children'] as they are the same
    //  variable. For instance, adding a child to $tree[$item['id']]['children]
    //  will be seen in 
    //  $tree[$parentid]['children'][<whatever index $item['id'] has>]['children]
    $tree[$parentid]['children'][] = &$tree[$item['id']];
 }
 $result = $tree['NULL']['children'];
 //always unset references
 unset($tree);