How do I get the category slug from wp_dropdown_categories

There is an argument walker for wp_dropdown_categories(). It accepts an instance of a custom walker, a class extending Walker_CategoryDropdown or the generic Walker.

Let’s create such a class. We have to change just one method.

class WPSE_Cat_Slug_Walker extends Walker_Category
{   function start_el( &$output, $category, $depth, $args, $id = 0 ) {
        $pad = str_repeat(' ', $depth * 3);

        $output .= "\t<option class=\"level-$depth\" value=\"".$category->term_id."\"";
        if ( $category->term_id == $args['selected'] )
            $output .= ' selected="selected"';
        $output .= '>';
        $output .= $pad.$category->slug; // The Slug!
        if ( $args['show_count'] )
            $output .= '&nbsp;&nbsp;('. $category->count .')';
        $output .= "</option>\n";
    }
}

Now we create an instance of our class …

$wpse_cat_slug_walker = new WPSE_Cat_Slug_Walker;

… and pass it to the dropdown:

$select = wp_dropdown_categories(
    array (
        'hierarchical' => 1,
        'hide_empty'   => 0,
        'echo'         => 0,
        'name'         => "field_$nextItem",
        'id'           => $selectID,
        'class'        => 'categoriesBox',
        'walker'       => $wpse_cat_slug_walker // the walker
    )
);

Note, this is completely untested, just an idea to show you the direction. 🙂