In order to display your categories in a dropdown menu, WordPress provide the wp_dropdown_categories() function. But if you want to display your tags in a dropdown as well, there’s no built in function. Let’s use wp_dropdown_categories() and modify it in order to be able to display tags in a dropdown menu.

Simply paste the following code where you want the dropdown menu to be displayed. Note that you can use your own taxonomy: Just modify the code on line 5 according to your needs.

 <h2><?php _e('Posts by Tags'); ?></h2> <form action="<?php bloginfo('url'); ?>/" method="get"> <div> <?php $select = wp_dropdown_categories('taxonomy=post_tag&show_option_none=Select tag&show_count=1&orderby=name&echo=0'); $select = preg_replace("#<select([^>]*)>#", "<select$1 onchange='return this.form.submit()'>", $select); echo $select; ?> <noscript><div><input type="submit" value="View" /></div></noscript> </div></form> 

Thanks to WP Lover for the code!