List stock filter

<?php

// gblijsten
/* Add In/Out of Stock Filter to Admin */
add_action('restrict_manage_posts', 'wpse45436_admin_posts_filter_restrict_manage_posts');

function wpse45436_admin_posts_filter_restrict_manage_posts()
{
    $type = 'product';
    if (isset($_GET['post_type'])) {
        $type = $_GET['post_type'];
    }

    // only add filter to post type you want

    if ('product' == $type) {

        // change this to the list of values you want to show
        // in 'label' => 'value' format

        $values = array(
            'Out of Stock' => 'outofstock',
            'In Stock' => 'instock',
        ); ?>

        <select name="Stock">

        <option value=""><?php
        _e('Show All Stock', 'wpse45436'); ?></option>

        <?php
        $current_v = isset($_GET['Stock']) ? $_GET['Stock'] : '';
        foreach ($values as $label => $value) {
            printf('<option value="%s"%s>%s</option>', $value, $value == $current_v ? ' selected="selected"' : '', $label);
        } ?>

        </select>

        <?php
    }
}

add_filter('parse_query', 'wpse45436_posts_filter');

function wpse45436_posts_filter($query)
{
    global $pagenow;
    $type = 'product';
    if (isset($_GET['post_type'])) {
        $type = $_GET['post_type'];
    }

    if ('product' == $type && is_admin() && $pagenow == 'edit.php' && isset($_GET['Stock']) && $_GET['Stock'] != '') {
        $query->query_vars['meta_key'] = '_stock_status';
        $query->query_vars['meta_value'] = $_GET['Stock'];
    }
}