title: Dropdown List multiselection in yii2
body: <p>I need multiselection in dropdown list. 
  This my dropdown list</p>
  
  <pre><code>&lt;?= $form-&gt;field($model, 'receiver_id')-&gt;dropDownList(
      ArrayHelper::map(User::find()-&gt;all(),'id','username'),
      ['prompt' =&gt; 'Select receiver...']
  ) ?&gt;
  </code></pre>
  
  <p>But it's only for one receiver. I want to choose some receivers and display them in textfield of this list separated by commas. Could you help with this problem?
  Thank you in advance for any help you can provide.</p>