Replace the `ver` query arg with the file's last modified timestamp (WordPress CSS/JS Cache Busting)

cache-busting-file-src.php

<?php
/**
 * Replace the `ver` query arg with the file's last modified timestamp
 *
 * @param  string $src URL to a file
 * @return string      Modified URL to a file
 */
function filter_cache_busting_file_src( $src = '' ) {
	global $wp_scripts;
	// If $wp_scripts hasn't been initialized then bail.
	if ( ! $wp_scripts instanceof WP_Scripts ) {
		return $src;
	}

	// Check if script lives on this domain. Can't rewrite external scripts, they won't work.
	$base_url = apply_filters( 'cache_busting_path_base_url', $wp_scripts->base_url, $src );
	if ( ! strstr( $src, $base_url ) ) {
		return $src;
	}

	// Remove the 'ver' query var: ?ver=0.1
	$src   = remove_query_arg( 'ver', $src );
	$regex = '/' . preg_quote( $base_url, '/' ) . '/';
	$path  = preg_replace( $regex, '', $src );
	$file  = null;

	// If the folder starts with wp- then we can figure out where it lives on the filesystem
	if ( strstr( $path, '/wp-' ) ) {
		$file = untrailingslashit( ABSPATH ) . $path;
	}
	if ( ! file_exists( $file ) ) {
		return $src;
	}

	$time_format     = apply_filters( 'cache_busting_path_time_format', 'Y-m-d_G-i' );
	$modified_time   = filemtime( $file );
	$timezone_string = get_option( 'timezone_string' );

	$dt = new DateTime( '@' . $modified_time );
	$dt->setTimeZone( new DateTimeZone( $timezone_string ) );
	$time = $dt->format( $time_format );
	$src  = add_query_arg( 'ver', $time, $src );
	return $src;
}

add_filter( 'script_loader_src', 'filter_cache_busting_file_src' );
add_filter( 'style_loader_src', 'filter_cache_busting_file_src' );