Filtering multiple taxonomies in admin post listing pages

taxonomy-filters.php

<?php

add_action( 'restrict_manage_posts', 'add_admin_taxonomy_filters' );
/**
 * Filtering for 2 taxonomies, 'genres', and 'colors'
 */
function add_admin_taxonomy_filters() {

	$screen = get_current_screen();
	if ( ! isset( $screen->id ) || $screen->id !== 'edit-post' )
		return;

	foreach ( array( 'genres', 'colors' ) as $taxonomy ) {
		build_taxonomy_dropdown( $taxonomy );
	}

}

/**
 * Create a select dropdown for a taxonomy
 */
function build_taxonomy_dropdown( $taxonomy ) {

	$taxonomy_object = get_taxonomy( $taxonomy );

	$args = array(
		'orderby'    =>  'name',
		'hide_empty' => 0,
	);
	$terms = get_terms( $taxonomy, $args );

	$selected = isset( $_REQUEST[ $taxonomy ] ) ? esc_attr( $_REQUEST[ $taxonomy ] ) : 0;

	$output = '<select name="'. $taxonomy .'" id="'. $taxonomy .'-dropdown" class="postform">';

	if ( empty( $terms ) ) {
		$output .= '<option value="-1" selected="selected">'. sprintf( __( 'No %s' ), $taxonomy_object->label ) .'</option>';
	} else {

		$output .= '<option value="0" '. selected( $selected, 0, false ) .'>'. sprintf( __( 'View All %s' ), $taxonomy_object->label ) .'</option>';

		foreach ( $terms as $term ) {
			$output .= '<option value="'. $term->slug .'" '. selected( $selected, $term->slug, false ) .'>'. $term->name .'</option>';
		}
	}

	$output .= '</select>';

	echo $output;
}