how to store check box value in database for yii2

I am new for yii2 .plz help me to solve this problem.

when i select the multiple checkbox values it show the field must be a string

thanks in advance

post some code please

it is my _form.php


use yii\helpers\Html;

use yii\widgets\ActiveForm;

/* @var $this yii\web\View */

/* @var $model app\models\Field */

/* @var $form yii\widgets\ActiveForm */


<div class="field-form">

&lt;?php &#036;form = ActiveForm::begin(); ?&gt;

&lt;?= &#036;form-&gt;field(&#036;model, 'name')-&gt;textInput(['maxlength' =&gt; true]) ?&gt;

&lt;?= &#036;form-&gt;field(&#036;model, 'gender')-&gt;textInput(['maxlength' =&gt; true]) ?&gt;

&lt;?= &#036;form-&gt;field(&#036;model, 'state')-&gt;textInput(['maxlength' =&gt; true]) ?&gt;

<?=$form->field($model, ‘skills’)->checkboxList([‘a’ => ‘Item A’, ‘b’ => ‘Item B’, ‘c’ => ‘Item C’])?>

&lt;div class=&quot;form-group&quot;&gt;

    &lt;?= Html::submitButton(&#036;model-&gt;isNewRecord ? 'Create' : 'Update', ['class' =&gt; &#036;model-&gt;isNewRecord ? 'btn btn-success' : 'btn btn-primary']) ?&gt;


&lt;?php ActiveForm::end(); ?&gt;