Taxonomy Term Previous / Next

January 04, 2023

Modified from function found here.

function get_tax_navigation( $taxonomy = 'category', $direction = '' ) {
	// Make sure we are on a taxonomy term/category/tag archive page, if not, bail
	if ( 'category' === $taxonomy ) {
		if ( !is_category() )
			return false;
	} elseif ( 'post_tag' === $taxonomy ) {
		if ( !is_tag() )
			return false;
	} else {
		if ( !is_tax( $taxonomy ) )
			return false;
	}

	// Make sure the taxonomy is valid and sanitize the taxonomy
	if (    'category' !== $taxonomy 
		 || 'post_tag' !== $taxonomy
	) {
		$taxonomy = filter_var( $taxonomy, FILTER_SANITIZE_STRING );
		if ( !$taxonomy )
			return false;

		if ( !taxonomy_exists( $taxonomy ) )
			return false;
	}

	// Get the current term object
	$current_term = get_term( $GLOBALS['wp_the_query']->get_queried_object() );

	// Get all the terms ordered by slug 
	$terms = get_terms( $taxonomy, ['orderby' => 'slug'] );

	// Make sure we have terms before we continue
	if ( !$terms ) 
		return false;

	// Because empty terms stuffs around with array keys, lets reset them
	$terms = array_values( $terms );

	// Lets get all the term id's from the array of term objects
	$term_ids = wp_list_pluck( $terms, 'term_id' );

	/**
	 * We now need to locate the position of the current term amongs the $term_ids array. \
	 * This way, we can now know which terms are adjacent to the current one
	 */
	$current_term_position = array_search( $current_term->term_id, $term_ids );

	// Set default variables to hold the next and previous terms
	$previous_term = '';
	$next_term     = '';

	// Get the previous term
	if (    'previous' === $direction 
		 || !$direction
	) {
		if ( 0 === $current_term_position ) {
			$previous_term = $terms[intval( count( $term_ids ) - 1 )];
		} else {
			$previous_term = $terms[$current_term_position - 1];
		}
	}

	// Get the next term
	if (    'next' === $direction
		 || !$direction
	) {
		if ( intval( count( $term_ids ) + 1 ) === $current_term_position ) {
			$next_term = $terms[0];
		} else {
			$next_term = $terms[$current_term_position + 1];
		}
	}

	$link = [];
	// Build the links
	if ( $previous_term ) 
		$link[] = '<a href="' . esc_url( get_term_link( $previous_term ) ) . '">Previous</a>';

	if ( $next_term ) 
		$link[] = '<a href="' . esc_url( get_term_link( $next_term ) ) . '">Next</a>';

	return implode( ' ...|... ', $link );
}