title: how to set default selected for select 2 multiple choice
body: <pre><code>    echo $form-&gt;field($model, 'user_id[]', [
            ])-&gt;widget(Select2::classname(), [
            'data' =&gt; $user_data,
            'pluginOptions' =&gt; [
                'allowClear' =&gt; true,
                'placeholder' =&gt; 'Select user...',
                'multiple' =&gt; true,
            ],
        ])-&gt;dropDownList($user_data,     [
            'multiple'=&gt;'multiple',
            'class'=&gt;'chosen-select input-md required',
           ]    )-&gt;label(Yii::t('app','User').' : '); 
  </code></pre>
  
  <p>By using the code above, I am able to load data from the database and display to user on the list of users, where they can type and select a user. Let's say i want to set 2 default selected values such as user_id = 1 and 2, how can i do so in the select 2?</p>