yii2 remove optgroup from dropdownlist

Question

How can i remove the optgroup from a checkbox field

<?= $form->field($model, 'survey_type')->dropDownList([$surveyTypeList],['prompt'=>'Select Survey Type','id'=>'Survey_Type_dropdown']) ?>

provides following html

<label class="control-label" for="Survey_Type_dropdown">Type</label>
<select id="Survey_Type_dropdown" class="form-control" name="Surveys[survey_type]">
<option value="">Select Survey Type</option>
<optgroup label="0">
<option value="2D">2D</option>
<option value="3D">3D</option>
</optgroup>
</select>

<div class="help-block"></div>

Answer 1

You have to send $surveyTypeList as a variable, not a array.

Just remove [].