Automatically cache bust scripts and styles

functions.php

<?php 

// wp_register_script( $handle, $src, $dependencies, $version, $includeInFooter )
wp_register_script( 'my-script', get_stylesheet_directory_uri() . '/js/scripts.min.js', array( 'jquery' ), filemtime( get_stylesheet_directory() . '/js/scripts.min.js' ), true );

notes.md

Notes:

• The version parameter is appended as a query string ( <script src="http://example.com/script.js?ver=1.0.0">)
• Appending a query string to a url will force a cache bust, telling the client to re-download the asset
• Using php's filemtime(), you can get a files last modified time as a unix timestamp and use that as the version number
• The result is that it will only cache bust when the file is modified