How to exclude categories from get_categories() select list inside a widget admin panel

Question

I have a widget that I need to add a category select list to. The end user should be able to select a category and I need to save the category ID with the widget.

I've hit a stumbling block in that I cannot get my exclude array to work. The excluded cats still show up in the dropdown. What am I doing wrong?

function form( $instance ) {
    $instance = wp_parse_args( (array) $instance, array( 'title' => '', 'text' => '', 'hide_title' => '', 'category_id' => '' ) );
    $title = format_to_edit($instance['title']);
    $text = format_to_edit($instance['text']);

    $hide_title = $instance['hide_title'] ? ' checked="checked"' : '';
    $category_id = $instance['category_id'] ? ' selected="selected"' : '';
    ?>
    <p>
        <label for="<?php echo $this->get_field_id( 'title' ); ?>">Title:</label>
        <input id="<?php echo $this->get_field_id( 'title' ); ?>" name="<?php echo $this->get_field_name( 'title' ); ?>" value="<?php echo $title; ?>" class="widefat" />
    </p>
    <select id="<?php echo $this->get_field_id( 'category_id' ); ?>" name="<?php echo $this->get_field_name( 'category_id' ); ?>"> 
     <option value=""><?php echo esc_attr(__('Select a Category')); ?></option> 
     <?php 
        $args = array('exclude' => array(get_cats()),'hide_empty' => 0 );
        $categories=get_categories($args); 
        foreach ($categories as $category) {
            $option = '<option value="'.$category->cat_ID.'">';
            $option .= $category->cat_name;
            $option .= '</option>';
            echo $option;
        }
     ?>
    </select>

function get_cats(){
    $exclude_cats = array(
        get_cat_ID('test1'),
        get_cat_ID('test2'),
        get_cat_ID('test3'),
        );
    return $exclude_cats;
}

Answer 1

Per http://codex.wordpress.org/Class_Reference/WP_Query you would need to prefix the numerical identifier of the array with a minus-sign, such as $query = new WP_Query( 'cat=-12,-34,-56' );, which would exclude the categories with the identifier 12, 34 and 56.

You could achieve this by changing the get_cats function into the following:

function get_cats(){
    $exclude_cats = array(
        get_cat_ID('test1'),
        get_cat_ID('test2'),
        get_cat_ID('test3'),
    );
    foreach ($exclude_cats as $item) {
        $array[] = '-'.$item;
    }
    return $array;
}

Which returns the array exactly as before, except each item has a minus-sign prepended to it.

Answer 2

get_cats() already returns an array, so using 'array' in here is too much:

'exclude' => array(get_cats()),

try:

'exclude' => get_cats(),