get_unique_meta

functions.php

/**
 * returns an array of unique items of meta_value, this was originally
 * built for a filter machanism.
 *
 * @param  string $post_type example: post
 * @param  string $meta_key  example: _wp_page_template
 * @return array
 */
function get_unique_meta($post_type, $meta_key)
{
	global $wpdb;

	$prepare = $wpdb->prepare("SELECT DISTINCT meta_value FROM $wpdb->postmeta m INNER JOIN $wpdb->posts p ON (m.post_id = p.ID) WHERE p.post_type = %s AND meta_key = %s ORDER BY meta_value ASC", $post_type, $meta_key);
	$result  = $wpdb->get_results($prepare);
	$return  = array();

	foreach ((array)$result as $item)
	{
		$return[] = $item->meta_value;
	}

	return $return;
}