WordPress post type permalinks

functions.php

<?php

/**
 * Modify Work permalink to add in category
 */
function mixd_work_permalink_setup( $post_link, $id = 0, $leavename = FALSE ) {
    // Test params are anywhere within the URL
    if ( strpos('%work_type%', $post_link) === 0 ) {
        return $post_link;
    } else {

        // Get the posts data
        $post = get_post($id);

        // If the $post data is an object and has a post_type of 'case-studies'
        if ( !is_object($post) || $post->post_type != 'work' ) {
            return $post_link;
        } else {

            // Get the $post's terms
            $terms = wp_get_object_terms($post->ID, 'work_type');

            // If there are no terms
            if ( !$terms ) {
                // change params to
                return str_replace('%work_type%', '', $post_link);
            } else {
                // change params to term slug
                return str_replace('%work_type%', $terms[0]->slug, $post_link);
            }
        }
    }
}

add_filter('post_type_link', 'mixd_work_permalink_setup', 1, 3);