how to empty the first select option with form api?

Question

I have made two select fields with the form api d7

and i need to make it required and the first option is null to force the user select both

i tried with #REQUIRED #EMPTY_VALUE OPTION but gives error unsupported operands

i see the validation function may help but my case have an ajax request based on the first select to get the values of the second

so i need to make it required and the first is null

the code answer

 $form['gov']=array('#title'=>'Gov','#name'=>'governorate'
,'#type'=>'select','#options'=>$options,'#ajax'=>array('callback'=>'ajax_callback',
'event'=>'change','wrapper'=>'replace'
  ,'method'=>'replace'));

 $form['city']=array("#title"=>'City',
'#name'=>'city','#type'=>'select'
,'#prefix' => '<div id="replace">',
'#suffix' => '</div>','#options'=>$op);

also the ajax request only happens one time

Answer 1

To make the input required you need to use #required => TRUE, and if you wants to add a empty default value you need to add a value to your #options like '' => t('Please select')

See Form API Reference

Answer 2

Add a empty value setting to the form array:

$form[$key] = array(
  ..
  '#empty_value' => '',
  ..
);

That will add a - Select - option to the list that will not pass a required validation if the user leaves it on - Select -.

Don't add an empty value to the #options array '' => t('Please select'). That will still pass a required validation if the user leaves that option selected.