[
        '' => 'Please Choose',
        '1' => 'item 1',
        '2' => 'item 2'
    ],
    [
        'onchange' => '$.post(Yii::$app->urlManager->createUrl . "users/A_action"), function(data) {
            $("#test_div").html(data)
        }'
    ]
);

控制器

 public function actionA_action() {
     $data = "TTT";
     return $data;
 }