cpt_unique_slug_by_tax.php

functions.php

<?php

add_filter( 'wp_unique_post_slug', 'cpt_unique_slug_by_tax', 10, 6 );	
function cpt_unique_slug_by_tax( $slug, $post_ID, $post_status, $post_type, $post_parent, $original_slug ) {
	if ( 'guide' === $post_type ) {
		global $wpdb;

		$post_tax = get_the_terms( $post_ID, 'guide_cat' );

		// Check if another CPT exists in the same taxonomy with the same name.
		$check_sql       = "SELECT p.post_name, t.name FROM $wpdb->posts as p "
			. "INNER JOIN $wpdb->term_relationships AS tr ON ( p.id = tr.object_id ) "
			. "INNER JOIN $wpdb->term_taxonomy AS tt ON (tr.term_taxonomy_id = tt.term_taxonomy_id) "
			. "INNER JOIN $wpdb->terms AS t ON (t.term_id = tt.term_id) "
			. "WHERE post_name = %s AND post_type IN ( %s, 'attachment' ) AND ID != %d AND post_parent = %d AND tt.taxonomy = %s LIMIT 1";

		$post_name_check = $wpdb->get_var( $wpdb->prepare( $check_sql, $original_slug, $post_type, $post_ID, $post_parent, $post_tax[0]->slug ) );

		if ( ! $post_name_check ) { // Return original slug if no matches in taxonomy.
			$slug = $original_slug;
		}
	}
	return $slug;
}