7-add-new-taxonomy-filter.php

7-add-new-taxonomy-filter.php

<?php
add_action( 'restrict_manage_posts', 'filter_posts_by_topics', 10, 2 );

function filter_posts_by_topics( $post_type, $which ) {

	/* Apply this only on a specific post type */
	if ( 'post' !== $post_type ) {
		return;
	}

	/* A list of taxonomy slugs to filter by */
	$taxonomies = array( 'topics' );

	foreach ( $taxonomies as $taxonomy_slug ) {

		$taxonomy_obj  = get_taxonomy( $taxonomy_slug ); /* Retrieve taxonomy data */
		$taxonomy_name = $taxonomy_obj->labels->name;
		$terms         = get_terms(  array(
			'taxonomy' => 'topics',
			'hide_empty' => false,
		) ); /* Retrieve taxonomy terms */

		/* Display filter HTML */
		echo '<select name="{$taxonomy_slug}" id="{$taxonomy_slug}" class="postform">';
		echo '<option value="">' . sprintf( esc_attr__( 'Show All %s', 'text_domain' ), esc_attr( $taxonomy_name ) ) . '</option>';
		foreach ( $terms as $term ) {
			printf(
				'<option value="%1$s" %2$s>%3$s (%4$s)</option>',
				esc_attr( $term->slug ),
				( ( isset( $_GET[$taxonomy_slug] ) && ( $_GET[$taxonomy_slug] === $term->slug ) ) ? ' selected="selected"' : '' ),
				esc_attr( $term->name ),
				esc_attr( $term->count )
			);
		}
		echo '</select>';
	}
}