How to use selected value in Html::dropDownList with optgroups in Yii2? - yii2

Question

in Yii2 I’m trying to choose a specific option with an Html::dropDownList that uses optgroup. I’m able to create the dropdown html just fine using a nested array but how do I select one of the choices? What do I use as the $selection value?
Here is the line of code that I'm currently using. If leave the selection blank, it all works fine, but as soon as I try to select a specific value, I can't get it to work.
echo Html::dropDownList(
'criteria',
['Page 1']['107141'],
[
'Page 1' => [
'107145' => 'Q1: some text',
'107141' => 'Q9: some text',
'107142' => 'Q10: some text',
'107143' => 'Q11: some text',
'107164' => 'Q14: some text',
],
'Page 2' => [
'107195' => 'some text',
],
],
[
'prompt' => [
'text' => 'Choose criteria',
'options' => []
],
'id' => 'criteriaSelector',
'class' => 'criteriaSelector',
'required' => 'required',
]
);
Here is the Yii documentation for the dropdownlist but I can't figure out what to do next:
https://www.yiiframework.com/doc/api/2.0/yii-helpers-basehtml#dropDownList()-detail
Thank you!

Answer 1

For some reason I kept thinking that the the selection needed to be an array element but, of course, since the dropdown option name has to be unique, the selection only needs to be a string. Here is the solution:
echo Html::dropDownList(
'criteria',
'107141',
[
'Page 1' => [
'107145' => 'Q1: some text',
'107141' => 'Q9: some text',
'107142' => 'Q10: some text',
'107143' => 'Q11: some text',
'107164' => 'Q14: some text',
],
'Page 2' => [
'107195' => 'some text',
],
],
[
'prompt' => [
'text' => 'Choose criteria',
'options' => []
],
'id' => 'criteriaSelector',
'class' => 'criteriaSelector',
'required' => 'required',
]
);