Create OptGroups for select dropdown (categories) in WordPress using selectize plugin

functions.php

<?php

class Top_level_Optgroup extends Walker_CategoryDropdown {

  var $optgroup = false;

  function start_el( &$output, $category, $depth = 0, $args = array(), $id = 0 ) {

    $pad = str_repeat('&nbsp;', $depth * 3);

    $cat_name = apply_filters('list_cats', $category->name, $category);
    if (0 == $depth) {
//    $this->optgroup = true;
      $output .= "<optgroup class=\"level-$depth\" label=\"".$cat_name."\" >";
    } else {
      $output .= "<option class=\"level-$depth\" value=\"".$category->term_id."\"";
      if ( $category->term_id == $args['selected'] )
              $output .= ' selected="selected"';
      $output .= '>';
      $output .= $pad.$cat_name;
      if ( $args['show_count'] )
              $output .= '&nbsp;&nbsp;('. $category->count .')';
      $output .= "</option>";
    }

  }

  function end_el(&$output,$object,$depth=0, $args = []) {
     if (0 == $depth/* && true == $this->optgroup*/) {
	 $output .= '</optgroup>';
      }
  }
}

// usage test
wp_dropdown_categories(
  array(
    'orderby' => 'name',
    'hide_empty' => 0,
    'show_count' => 1,
    'show_option_none' => 'Select one',
    'class' => 'cat',
    'hierarchical' => 1,
    'name' => 'cat',
    'walker' => new Top_level_Optgroup
  )
);
?>

Works perfectly
Just a litle note: with latest versions of Wordpress, the end_el() should have default parameters values and the fourth one should be like so:

function end_el(&$output,$object,$depth=0, $args = []) {
     if (0 == $depth/* && true == $this->optgroup*/) {
	 $output .= '</optgroup>';
      }
}

Perfect! Thanks @iBazzo i edited the snipppet

Another little improvment..
$output .= "<option class=\"level-$depth\" value=\"".$category->term_id."\"";
with this code we assume that the value should be always the category ID, but in some cases we would want the option value to be what ever we put un the argument "value_field" passed to wp_dropdown_categories()
so, it would be better to set it dynamically, according to user $args put to the function, like this:

$output .= "<option class=\"level-$depth\" value=\"".$category->{$args['value_field']}."\"";
         
if ( $category->{$args['value_field']} == $args['selected'] ) {
     $output .= ' selected="selected"';
}

and again thanks @dsebao for the useful snippet
hope this helps some one