<?php
// Add the Meta Box
function vs_add_excerpt_meta_box() {
    add_meta_box(
        'excerpt_meta_box', // $id
        'Excerpt Meta Box', // $title
        'vs_show_excerpt_meta_box', // $callback
        'post', // $page
        'normal', // $context
        'high'); // $priority
}
add_action('add_meta_boxes', 'vs_add_excerpt_meta_box');

// Field Array
$prefix = 'custom_';
$custom_meta_fields = array(
    array(
        'label' => 'Custom post teaser',
        'desc'  => 'Replace the default excerpt the text of your choice',
        'id'    => $prefix.'excerpt',
        'type'  => 'textarea'
        )
);

// The Callback
function vs_show_excerpt_meta_box() {
global $custom_meta_fields, $post;
// Use nonce for verification
echo '<input type="hidden" name="excerpt_meta_box_nonce" value="'.wp_create_nonce(basename(__FILE__)).'" />';

    // Begin the field table and loop
    echo '<table class="form-table">';
    foreach ($custom_meta_fields as $field) {
        // get value of this field if it exists for this post
        $meta = get_post_meta($post->ID, $field['id'], true);
        // begin a table row with
        echo '<tr>
                <th><label for="'.$field['id'].'">'.$field['label'].'</label></th>
                <td>';
                switch($field['type']) {
                    // case items will go here
                    // text
case 'text':
    echo '<input type="text" name="'.$field['id'].'" id="'.$field['id'].'" value="'.$meta.'" size="30" />
        <br /><span class="description">'.$field['desc'].'</span>';
break;
// textarea
case 'textarea':
    echo '<textarea name="'.$field['id'].'" id="'.$field['id'].'" cols="60" rows="4">'.$meta.'</textarea>
        <br /><span class="description">'.$field['desc'].'</span>';
break;
// checkbox
case 'checkbox':
    echo '<input type="checkbox" name="'.$field['id'].'" id="'.$field['id'].'" ',$meta ? ' checked="checked"' : '','/>
        <label for="'.$field['id'].'">'.$field['desc'].'</label>';
break;
// select
case 'select':
    echo '<select name="'.$field['id'].'" id="'.$field['id'].'">';
    foreach ($field['options'] as $option) {
        echo '<option', $meta == $option['value'] ? ' selected="selected"' : '', ' value="'.$option['value'].'">'.$option['label'].'</option>';
    }
    echo '</select><br /><span class="description">'.$field['desc'].'</span>';
break;
                } //end switch
        echo '</td></tr>';
    } // end foreach
    echo '</table>'; // end table
}

// Save the Data
function vs_save_excerpt_meta($post_id) {
    global $custom_meta_fields;

    // verify nonce
    if (!wp_verify_nonce($_POST['excerpt_meta_box_nonce'], basename(__FILE__)))
        return $post_id;
    // check autosave
    if (defined('DOING_AUTOSAVE') && DOING_AUTOSAVE)
        return $post_id;
    // check permissions
    if ('page' == $_POST['post_type']) {
        if (!current_user_can('edit_page', $post_id))
            return $post_id;
        } elseif (!current_user_can('edit_post', $post_id)) {
            return $post_id;
    }

    // loop through fields and save the data
    foreach ($custom_meta_fields as $field) {
        $old = get_post_meta($post_id, $field['id'], true);
        $new = $_POST[$field['id']];
        if ($new && $new != $old) {
            update_post_meta($post_id, $field['id'], $new);
        } elseif ('' == $new && $old) {
            delete_post_meta($post_id, $field['id'], $old);
        }
    } // end foreach
}
add_action('save_post', 'vs_save_custom_meta');
?>