Add termmeta support to WordPress including a termmeta table and all the standard WordPress metadata functions

termmeta.php

<?php

/**
 * Setup the termmeta table
 *
 * Use <code>add_theme_support( 'term-meta' );</code> to enable support for term meta 
 */
function hm_add_term_meta_table() {

	global $wpdb;

	if ( ! current_theme_supports( 'term-meta' ) )
		return false;

	hm_create_term_meta_table();

	$wpdb->tables[] = 'termmeta';
	$wpdb->termmeta = $wpdb->prefix . 'termmeta';

}
add_action( 'init', 'hm_add_term_meta_table' );

/**
 * Create the termmeta table if it doesn't exist
 *
 * @todo should check if the table exists directly rather than relying on an option
 */
function hm_create_term_meta_table() {
	
        global $wpdb;

	// check if the table already exists
	if ( get_option( 'hm_created_term_meta_table' ) )
		return false;

	$wpdb->query( "
		CREATE TABLE `{$wpdb->prefix}termmeta` (
		  `meta_id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
		  `term_id` bigint(20) unsigned NOT NULL DEFAULT '0',
		  `meta_key` varchar(255) DEFAULT NULL,
		  `meta_value` longtext,
		  PRIMARY KEY (`meta_id`),
		  KEY `term_id` (`term_id`),
		  KEY `meta_key` (`meta_key`)
		) ENGINE=MyISAM AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;" );

	update_option( 'hm_created_term_meta_table', true );

	return true;
}

if ( ! function_exists( 'add_term_meta' ) ) :
/**
 * Add meta data field to a term.
 *
 * @param int $term_id term_id.
 * @param string $key Metadata name.
 * @param mixed $value Metadata value.
 * @param bool $unique Optional, default is false. Whether the same key should not be added.
 * @return bool False for failure. True for success.
 */
function add_term_meta( $term_id, $meta_key, $meta_value, $unique = false ) {
    return add_metadata( 'term', $term_id, $meta_key, $meta_value, $unique );
}
endif;

if ( ! function_exists( 'delete_term_meta' ) ) :
/**
 * Remove metadata matching criteria from a term.
 *
 * You can match based on the key, or key and value. Removing based on key and
 * value, will keep from removing duplicate metadata with the same key. It also
 * allows removing all metadata matching key, if needed.
 *
 * @param int $term_id term_id
 * @param string $meta_key Metadata name.
 * @param mixed $meta_value Optional. Metadata value.
 * @return bool False for failure. True for success.
 */
function delete_term_meta( $term_id, $meta_key, $meta_value = '' ) {
    return delete_metadata( 'term', $term_id, $meta_key, $meta_value );
}
endif;

if ( ! function_exists( 'get_term_meta' ) ) :
/**
 * Retrieve term meta field for a term.
 *
 * @param int $term_id term_id.
 * @param string $key The meta key to retrieve.
 * @param bool $single Whether to return a single value.
 * @return mixed Will be an array if $single is false. Will be value of meta data field if $single
 * is true.
 */
function get_term_meta( $term_id, $key, $single = false ) {
    return get_metadata( 'term', $term_id, $key, $single );
}
endif;

if ( ! function_exists( 'update_term_meta' ) ) :
/**
 * Update term meta field based on term_id.
 *
 * Use the $prev_value parameter to differentiate between meta fields with the
 * same key and term_id.
 *
 * If the meta field for the term does not exist, it will be added.
 *
 * @param int $term_id term ID.
 * @param string $key Metadata key.
 * @param mixed $value Metadata value.
 * @param mixed $prev_value Optional. Previous value to check before removing.
 * @return bool False on failure, true if success.
 */
function update_term_meta( $term_id, $meta_key, $meta_value, $prev_value = '' ) {
    return update_metadata( 'term', $term_id, $meta_key, $meta_value, $prev_value );
}
endif;

if ( ! function_exists( 'get_term_custom' ) ) :
/**
 * Retrieve term meta fields, based on term_id.
 *
 * The term meta fields are retrieved from the cache, so the function is
 * optimized to be called more than once. It also applies to the functions, that
 * use this function.
 *
 * @param int $term_id term_id
 * @return array
 */
 function get_term_custom( $term_id = 0 ) {

    $term_id = (int) $term_id;

    if ( ! wp_cache_get( $term_id, 'term_meta' ) )
        update_termmeta_cache( $term_id );

    return wp_cache_get( $term_id, 'term_meta' );
}
endif;

if ( ! function_exists( 'update_termmeta_cache' ) ) :
/**
* Updates metadata cache for list of term_ids.
*
* Performs SQL query to retrieve the metadata for the term_ids and updates the
* metadata cache for the terms. Therefore, the functions which call this
* function do not need to perform SQL queries on their own.
*
* @param array $term_ids List of term_ids.
* @return bool|array Returns false if there is nothing to update or an array of metadata.
*/
function update_termmeta_cache( $term_ids ) {
    return update_meta_cache( 'term', $term_ids );
}
endif;

Do you have a related example of adding some UI to make use of this on the WordPress taxonomy term edit page? In your post – How we built marketing.yell.com – you mention

The design of the Category archive pages required us to add some custom fields to the default edit term page, in addition to title, slug, parent and description we added the ability to define the header image.

Could you put up a Gist that shows how you did this, specifically the header image?

Thanks!

Only just seen this comment @emzo, sorry I missed it! Did you ever get it working?