wp_dropdown_categories multi select

functions.php

<?php
function wp_dropdown_categories_pk($name, $taxonomy, $selected = array())
{
    $select = wp_dropdown_categories(array(
        'name' => $name,
        'show_option_none' => '- Category -',
        'option_none_value' => '-1',
        'taxonomy' => $taxonomy,
        'hierarchical' => 1,
        'echo' => false
    ));

    if (is_array($selected))
        foreach ($selected as $sel)
            if (!empty($sel) && $sel != '-1')
                $select = str_replace('value="' . $sel . '"', 'value="' . $sel . '" selected', $select);

    return $select;
}

// Active Multi Select
function dropdown_filter($output, $r)
{
    $output = preg_replace('/<select (.*?) >/', '<select $1 size="5" multiple>', $output);
    return $output;
}

add_filter('wp_dropdown_cats', 'dropdown_filter', 10, 2);

index.php

Categories: 
<?php echo wp_dropdown_categories_pk('multiple_categories[]', 'category', array(10,12)); ?>