<?php
    //First drop-down
    echo $form->field($model, 'client')->dropDownList($Client,
     ['prompt'=>'-Select Client-',
      'onchange'=>'
        $.post
        (
            "'.urldecode(
      Yii::$app->urlManager->createUrl
      ('leads/lists&id=')).'"+$(this).val(), function( data )
         {
          $( "select#staff_id" ).html( data );
        });
    ']); ?>

     // depend dropdown
    <?php echo $form->field($model, 'staff')
          ->dropDownList
           (
                 ['prompt'=>'-Choose a Sub Category-'],
                 ['id'=>'staff_id','value'=>$Staff]
            );
    ?>

public function actionLists($id)
{
    $sql = "select * from staff  where client='$id' ";
   //exit;
    $models = Staff::findBySql($sql)->asArray()->all();
    //echo "<pre>";print_r($model);exit;

    if(sizeof($models) >0){
        echo "<option>-Choose a Sub Category-</option>";
        foreach($models as $model){
            echo "<option value='".$model['id']."'>".$model['fname']."</option>";
        }
    }
    else{
        echo "<option>-Choose a Sub Category-</option><option></option>";
    }

}