如何在基于Yii2的下拉列表中添加自定义选项？

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-

$book_list = Books::find()
            ->where(['Status' => 1])
            ->orderBy('BookName')
            ->all();

$listBook=ArrayHelper::map($book_list,'BookName','BookName');

<?= $form->field($model, 'BookName')->dropDownList($listBook, [
'prompt' => 'Select'], 
['label'=>'']
)?>

And it generating dropdown list like following-

<option value="">Select</option>
<option value="Biology">Biology</option>
<option value="Mathematics">Mathematics</option>
<option value="Physics">Physics</option>

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.

<option value="">Select</option>
<option value="Biology">Biology</option>
<option value="Mathematics">Mathematics</option>
<option value="Physics">Physics</option>
<option value="Other">Other</option>

How can I achieve this?

Please try like this,

$listBook=ArrayHelper::map($book_list,'BookName','BookName');
$listBook['Other'] = 'Other';

Sorry for delay, Below is the correct answer.

$listBook=ArrayHelper::map($book_list,'BookName','BookName');
$listBookNew = array_merge($listBook, array('Other'=>'Other'));
<?= $form->field($model, 'BookName')->dropDownList($listBookNew, [
'prompt' => 'Select'], 
['label'=>'']
)?>

Let me know if it works