<？php wp_dropdown_categories（）; ？>更改循环类别

I've been googling for ages to no avail.

I need to filter a loop based on the selection of a dropdown. This should automatically filter the loop below to show the corresponding category options.

<?php wp_dropdown_categories( array('lowest_fares' => $taxonomy) ); ?>
<?php $loop = new WP_Query( array('post_type' => 'lowest_fares', 'orderby' => 'post_id', 'order' => 'ASC') ); ?>
      <?php while( $loop->have_posts() ) : $loop->the_post(); ?>
<!--ADVANCED CUSTOM FIELS STUFF HERE-->
<?php endwhile; ?>

Any help would be greatly appreciated.