<?php
add_action(
    'widgets_init'
,   array ('Toscho_Mini_Widget', 'register' )
);
class Toscho_Mini_Widget extends WP_Widget
{
    function Toscho_Mini_Widget()
    {
        $this->WP_Widget(__CLASS__, __CLASS__);
    }
    function widget($args, $instance)
    {
        print 'Juhu, hat geklappt!';
    }
    function register()
    {
        register_widget(__CLASS__);
    }
}

<?php
add_action(
    'widgets_init'
,   array ('Restricted_Latest_Posts_Widget', 'register' )
);
/**
 * Restricted Recent Posts widget class.
 *
 * Restricts the list of recent posts to selected categories.
 * Derivate of the default widget.
 *
 * @todo Select post types
 * @todo Select taxonomy
 */
class Restricted_Latest_Posts_Widget extends WP_Widget
{
    // Adjust this to your needs.
    var
        $max_posts = 50
    ,   $wid_name  = 'widget_restricted_recent_posts';
    /**
     * Constructor.
     */
    function Restricted_Latest_Posts_Widget()
    {
        $widget_ops = array(
            'classname'   => $this->wid_name
        ,   'description' => 'Letzte Beiträge per Kategorie ausgewählt.
                                Mit Komma trennen.'
        );
        $this->WP_Widget(
            'restricted-recent-posts'
        ,   'Restricted Recent Posts'
        ,   $widget_ops
        );
        $this->alt_option_name = $this->wid_name;
        add_action( 'save_post',
            array(&$this, 'flush_widget_cache') );
        add_action( 'deleted_post',
            array(&$this, 'flush_widget_cache') );
        add_action( 'switch_theme',
            array(&$this, 'flush_widget_cache') );
    }
    /**
     * Called statically to register the widget.
     *
     * @return void.
     */
    function register()
    {
        register_widget(__CLASS__);
    }
    function widget($args, $instance)
    {
        $cache = wp_cache_get($this->wid_name, 'widget');
        if ( ! is_array($cache) )
        {
            $cache = array();
        }
        if ( isset ( $cache[ $args['widget_id'] ] ) )
        {
            echo $cache[ $args['widget_id'] ];
            return;
        }
        ob_start();
        extract($args);
        $title        = empty ( $instance['title'] )
                        ? 'Restricted Recent Posts'
                        : $instance['title'];
        $title        = apply_filters('widget_title', $title);
        $number       = $this->sanitize_number($instance['number']);
        $include_cats = trim($instance['include_cats']);
        $exclude_cats = trim($instance['exclude_cats']);
        $query_args   = array(
            'showposts'        => $number
        ,   'nopaging'         => 0
        ,   'post_status'      => 'publish'
        ,   'caller_get_posts' => 1
        );
        if ( ! empty ( $include_cats ) )
        {
            $query_args['category__in']
                = $this->catlist_to_array($include_cats);
        }
        elseif ( ! empty ( $exclude_cats ) )
        {
            $query_args['category__not_in']
                = $this->catlist_to_array($exclude_cats);
        }
        $r = new WP_Query($query_args);
        if ( $r->have_posts() )
        {
            print $before_widget
                . ( $title
                        ? $before_title . $title . $after_title : ''
                    )
                . '<ul>';
            while ( $r->have_posts() )
            {
                $r->the_post();
                print '<li><a href="' . get_permalink() . '">'
                    . get_the_title() . '</a></li>';
            }
            print "</ul>$after_widget";
            // Restore global post data stomped by the_post().
            wp_reset_query();
        }
        $cache[ $args['widget_id'] ] = ob_get_flush();
        wp_cache_add($this->wid_name, $cache, 'widget');
    }
    function update( $new_instance, $old_instance )
    {
        $instance                 = $old_instance;
        $instance['title']        = strip_tags($new_instance['title']);
        $instance['number']       =
                $this->sanitize_number($new_instance['number']);
        $instance['exclude_cats'] = $new_instance['exclude_cats'];
        $instance['include_cats'] = $new_instance['include_cats'];
        $this->flush_widget_cache();
        $alloptions = wp_cache_get( 'alloptions', 'options' );
        if ( isset ( $alloptions[$this->wid_name] ) )
        {
            delete_option($this->wid_name);
        }
        return $instance;
    }
    function flush_widget_cache()
    {
        wp_cache_delete($this->wid_name, 'widget');
    }
    function form( $instance )
    {
        $title = isset ( $instance['title'] )
                ? esc_attr($instance['title']) : '';
        if (   ! isset ( $instance['number'])
            || ! $number = (int) $instance['number']
        )
        {
            $number = 5;
        }
        $incats = isset ( $instance['include_cats'] )
            ? esc_attr($instance['include_cats']) : '';
        $excats = isset ( $instance['exclude_cats'] )
            ? esc_attr($instance['exclude_cats']) : '';
        ?>
        <p>
            <label for="<?php
                echo $this->get_field_id('title');
            ?>">
                <?php
                    _e('Title:');
                ?>
            </label>
            <input class="widefat"
                id="<?php
                    echo $this->get_field_id('title');
                ?>"
                name="<?php
                    echo $this->get_field_name('title');
                ?>"
                type="text"
                value="<?php
                    echo $title;
                ?>"
            />
        </p>
        <p>
            <label for="<?php
                echo $this->get_field_id('number');
            ?>">
                Artikelmenge
            </label>
            <input id="<?php
                echo $this->get_field_id('number');
            ?>"
                name="<?php
                    echo $this->get_field_name('number');
                ?>"
                type="text"
                value="<?php
                    echo $number;
                ?>"
                size="3" />
            <small>(maximal <?php
                print $this->max_posts;
            ?>)</small>
        </p>
        <p>
            <label for="<?php
                echo $this->get_field_id('include_cats');
            ?>">
                Auf Kategorien beschränken:
            </label>
            <input id="<?php
                echo $this->get_field_id('include_cats');
            ?>"
                name="<?php
                    echo $this->get_field_name('include_cats');
                ?>"
                type="text"
                value="<?php
                    echo $incats;
                ?>"
                size="10"
            />
            <br /><strong>oder</strong><br />
            <label for="<?php
                echo $this->get_field_id('exclude_cats');
            ?>">
                Kategorien ausschließen:
            </label>
            <input id="<?php
                echo $this->get_field_id('exclude_cats');
            ?>"
                name="<?php
                    echo $this->get_field_name('exclude_cats');
                ?>"
                type="text"
                value="<?php
                    echo $excats;
                ?>"
                size="10"
            />
        </p>
        <?php
    }
    /**
     * Guarantee a number between 1 and max_posts.
     *
     * @return int
     */
    function sanitize_number($nr)
    {
        $nr = (int) $nr;
        if ( $nr > $this->max_posts )
        {
            return $this->max_posts;
        }
        if ( 0 == $nr )
        {
            return 1;
        }
        return $nr;
    }
    /**
     * Used by widget() to fill the category parameter.
     *
     * @param string $catlist
     * @return array
     */
    function catlist_to_array($catlist)
    {
        $tmp_cats = explode(',', $catlist);
        $new_cats = array ();
        foreach ( $tmp_cats as $cat )
        {
            $new_cats[] = (int) trim($cat);
        }
        return $new_cats;
    }
}