Override/extend post save

6-extending-post-save.php

<?php
add_action( 'save_post', 'update_post_title' );

function update_post_title( $post_id ){
	global $wpdb;
	$post_type = get_post_type( $post_id );
	$title     = '';

	if ( $post_type === 'store' ) {
		$name  = get_post_meta( $post_id, 'name', true );
		$title = $name;
	}
	if ($post_type === 'register') {
		$first_name = get_post_meta( $post_id, 'first_name', true );
		$vat .      = get_post_meta( $post_id, 'vat', true );
		$title      = '##' . $first_name . '## ' . $vat;
	}

	if( $title !== '' ){
		$post             = get_post( $post_id );
		$post->post_title = $title;
		$post->post_name  = sanitize_title( $title );

		remove_action( 'save_post', 'update_post_title' );
		wp_update_post( $post );
		add_action( 'save_post', 'update_post_title' );
	}
}