Yii2 dropdown selected value

Try this.

$model->userid=$id;
$form->field($model, 'userid')
->dropDownList(...)
->label('');

Basically, you affect the options (your <option> elements) by using the value attribute's actual value as the array key in the dropDownList options array.

So in this case I have an array of states and the value attributes have the state abbreviation, for example value="FL". I'm getting my selected state from the Address table, which stores the abbreviation, so all I have to do is use that as my array key in the options array:

echo $form->field($model, 'state')->dropDownList($listData, ['prompt'=>'Select...', 'options'=>[$address->state=>["Selected"=>true]]]);

The documentation spells it out: http://www.yiiframework.com/doc-2.0/yii-helpers-basehtml.html#dropDownList()-detail

$model->userid = $_GET['cid'];
$form->field($model, 'userid')
->dropDownList( 
      $items,                   //Flat array('id'=>'val')
['prompt'=>'']                  //options
)->label('');

i hope this will help you

$form->field($model, 'userid')
    ->dropDownList(
          [User::getUser()],
          //[ArrayHelper::map(User::find()->where('id' => $id)->all(), 'id', 'name')],
          ['prompt'=>'Select a user','id'=>'user_dropdown'],    
          ['options' =>
                    [                        
                      $id => ['selected' => true]
                    ]
          ]

        )->label('');