composer require kartik-v/yii2-widget-select2 "@dev"

//如果你的表单是ActiveForm，请使用

use kartik\select2\Select2; 
//$data是键值对数组哦，key-value ,下面所声明的所有$data均为键值对数组，以该数组为例 
$data = [2 => 'widget', 3 => 'dropDownList', 4 => 'yii2']; 
echo $form->field($model, 'title')->widget(Select2::classname(), [ 
  'data' => $data, 
  'options' => ['placeholder' => '请选择 ...'], 
]);

//如果你的表单是非ActiveForm,可以参考下面的

use kartik\select2\Select2; 
echo Select2::widget([ 'name' => 'title', 
  'data' => $data, 
  'options' => ['placeholder' => '请选择...'] 
]);

use kartik\select2\Select2; 
echo Select2::widget([ 
  'name' => 'title', 
  'value' => 2, 
  'data' => $data, 
  'options' => ['placeholder' => '请选择...'] 
]);

echo $form->field($model, 'title')->widget(Select2::classname(), [ 
  'data' => $data, 
  'options' => ['multiple' => true, 'placeholder' => '请选择 ...'], 
]);
#多选的添加默认值同上

// view层
use kartik\select2\Select2;
use yii\web\JsExpression;

<?php
  echo $form->field($model, 'title')->widget(Select2::classname(), [
    'options' => ['placeholder' => '请输入标题名称 ...'],
    'pluginOptions' => [
      'placeholder' => 'search ...',
      'allowClear' => true,
      'language' => [
        'errorLoading' => new JsExpression("function () { return 'Waiting...'; }"),
      ],
      'ajax' => [
        'url' => '这里是提供数据源的接口',
        'dataType' => 'json',
        'data' => new JsExpression('function(params) { return {q:params.term}; }')
      ],
      'escapeMarkup' => new JsExpression('function (markup) { return markup; }'),
      'templateResult' => new JsExpression('function(res) { return res.text; }'),
      'templateSelection' => new JsExpression('function (res) { return res.text; }'),
    ],
  ]);
?>

//controller层
public function actionSearchTitle ($q)
{
  \Yii::$app->response->format = \yii\web\Response::FORMAT_JSON;
  $out = ['results' => ['id' => '', 'text' => '']];
  if (!$q) {
    return $out;
  }

  $data = Article::find()
        ->select('id, title as text')
        ->andFilterWhere(['like', 'title', $q])
        ->limit(50)
        ->asArray()
        ->all();
        
  $out['results'] = array_values($data);

  return $out;
}