title: How to add custom option in Yii2 based dropdown list?
body: <p>I can create model based dropdown list in Yii2 using the ArrayHelper::map(). But I am not sure about the process to add a new select option in the dropdown list. Bellow I have added my code-</p>
  
  <pre><code>$book_list = Books::find()
              -&gt;where(['Status' =&gt; 1])
              -&gt;orderBy('BookName')
              -&gt;all();
  
  $listBook=ArrayHelper::map($book_list,'BookName','BookName');
  
  &lt;?= $form-&gt;field($model, 'BookName')-&gt;dropDownList($listBook, [
  'prompt' =&gt; 'Select'], 
  ['label'=&gt;'']
  )?&gt;
  </code></pre>
  
  <p>And it generating dropdown list like following-</p>
  
  <pre><code>&lt;option value=""&gt;Select&lt;/option&gt;
  &lt;option value="Biology"&gt;Biology&lt;/option&gt;
  &lt;option value="Mathematics"&gt;Mathematics&lt;/option&gt;
  &lt;option value="Physics"&gt;Physics&lt;/option&gt;
  </code></pre>
  
  <p>I want to add a new option called "OTHER" at the end of the dropdown list. I am using order by BookName so "OTHER" will be out of the sorting. It will always display in bottom/top of the dropdown list.</p>
  
  <pre><code>&lt;option value=""&gt;Select&lt;/option&gt;
  &lt;option value="Biology"&gt;Biology&lt;/option&gt;
  &lt;option value="Mathematics"&gt;Mathematics&lt;/option&gt;
  &lt;option value="Physics"&gt;Physics&lt;/option&gt;
  &lt;option value="Other"&gt;Other&lt;/option&gt;
  </code></pre>
  
  <p>How can I achieve this? </p>