如何在Yii2应用程序中的多个选择下拉列表中显示所选值？

I am working on Yii2. I am creating multiple select drop down using custom array like this.

In controller file:

$all_groups = Groups::find()->where(['=','group_created_by',$id])->orwhere(new Expression('FIND_IN_SET(:id_to_find, group_managers)'))->addParams([':id_to_find' => $id])->all(); // fetch all values

$selected_groups  = Groups::find()->where(['=','group_users',$updateId])->orwhere(new Expression('FIND_IN_SET(:id_to_find, group_users)'))->addParams([':id_to_find' => $updateId])->all(); // getting selected values

$all_groups_array = [];

 foreach ($all_groups as $group) {
     $all_groups_array[$group->id] = ucfirst($group->group_name);
 }

On render data on the view:

return $this->render('mngr_userupdate', [
                        'model' => $model,
                        'all_groups_array'=>$all_groups_array,
                        'case'=>$case,
                        'email_error' => 'false',
                        'applied_email' =>   '' ,
                      ]);

so it is creating array like this:

Array
(
    [11] => Mngr1 group
    [14] => Mngr 11 Group
)

In vies file:

 <?= $form->field($model, 'group_user[]')->dropDownList($all_groups_array,['multiple' => 'multiple']) ?>

It is working fine on create form for data insertion. but how to create array using which I can display selected values on update form.

Edit:

I just found that if I use it like

<?= $form->field($model, 'group_user[]')->dropDownList($all_groups_array,['multiple' => 'multiple', 'options'=>['14'=>["Selected"=>true],'11' => ["Selected"=>true]]]); ?>

then it will start display values as selected. i.e. I have to create array like

[
'14'=>["Selected"=>true],
'11' => ["Selected"=>true]
]

For this I am using loop like as follows:

foreach ($selected_groups as $key => $value) {
           $sel_groups_array[$value] = '' // what should be there or else
          }

How can I create this array using loop?