Shows the taxonomy selection in admin as <select> box

taxonomy_select.php

<?php
// Author: Jari Pennanen
// License: Public Domain
// 2014-04-16
 
/**
 * Display taxonomy selection in admin as select box
 *
 * @param WP_Post $post
 * @param array $box
 */
function my_admin_taxonomy_select_meta_box($post, $box) {
    $defaults = array('taxonomy' => 'category');
    
    if (!isset($box['args']) || !is_array($box['args']))
        $args = array();
    else
        $args = $box['args'];
    
    extract(wp_parse_args($args, $defaults), EXTR_SKIP);
    
    $tax = get_taxonomy($taxonomy);
    $selected = wp_get_object_terms($post->ID, $taxonomy, array('fields' => 'ids'));
    $hierarchical = $tax->hierarchical;
    ?>
    <div id="taxonomy-<?php echo $taxonomy; ?>" class="selectdiv">
        <?php if (current_user_can($tax->cap->edit_terms)): ?>
        <?php 
            if ($hierarchical) {
                wp_dropdown_categories(array(
                   'taxonomy' => $taxonomy,
                   'class' => 'widefat',
                   'hide_empty' => 0, 
                   'name' => "tax_input[$taxonomy][]",
                   'selected' => count($selected) >= 1 ? $selected[0] : '',
                   'orderby' => 'name', 
                   'hierarchical' => 1, 
                   'show_option_all' => " "
                ));
            } else {
                ?>
                <select name="<?php echo "tax_input[$taxonomy][]"; ?>" class="widefat">
                    <option value="0"></option>
                    <?php foreach (get_terms($taxonomy, array('hide_empty' => false)) as $term): ?>
                        <option value="<?php echo esc_attr($term->slug); ?>" <?php echo selected($term->term_id, count($selected) >= 1 ? $selected[0] : ''); ?>><?php echo esc_html($term->name); ?></option>
                    <?php endforeach; ?>
                </select>
                <?php 
            }
        ?>
        <?php endif; ?>
    </div>
    <?php
}
 
// Usage example
register_taxonomy("mytaxonomy", "page", array(
    "label" => "My Taxonomy",
    "meta_box_cb" => "my_admin_taxonomy_select_meta_box",
));