<?php 
/**
 * Replaces the default excerpt editor with TinyMCE.
 */
class T5_Richtext_Excerpt
    {
        /**
         * Replaces the meta boxes.
         *
         * @return void
         */
        public static function switch_boxes()
            {
                if ( ! post_type_supports( $GLOBALS['post']->post_type, 'excerpt' ) )
                    return;

                remove_meta_box(
                    'postexcerpt',    // ID
                    '',               // Screen, empty to support all post types
                    'normal'          // Context
                );

                add_meta_box(
                    'postexcerpt2'     // Reusing just 'postexcerpt' doesn't work.
                    ,   __( 'Excerpt' )    // Title
                    ,   array ( __CLASS__, 'show' ) // Display function
                    ,   array()              // Screen, we use all screens with meta boxes.
                    ,   'normal'          // Context
                    ,   'core'            // Priority
                );
            }

        /**
         * Output for the meta box.
         *
         * @param  object $post
         * @return void
         */
        public static function show( $post )
            {
                ?>
                <label class="screen-reader-text" for="excerpt"><?php
                _e( 'Excerpt' )
                ?></label>
                <?php
                // We use the default name, 'excerpt', so we don’t have to care about
                // saving, other filters etc.
                wp_editor(
                    self::unescape( $post->post_excerpt ),
                    'excerpt',
                    array (
                        'textarea_rows' => 15
                        ,   'media_buttons' => FALSE
                        ,   'teeny'         => TRUE
                        ,   'tinymce'       => TRUE
                    )
                );
            }

        /**
         * The excerpt is escaped usually. This breaks the HTML editor.
         *
         * @param  string $str
         * @return string
         */
        public static function unescape( $str )
            {
                return str_replace(
                    array ( '&lt;', '&gt;', '&quot;', '&amp;', '&nbsp;', '&amp;nbsp;' ),   
                    array ( '<',    '>',    '"',      '&',     ' ', ' ' ),
                    $str
                );
            }
    }