Add timestamp to CSS and JS URLs to cachebust

theme/footer.php

@@ -1 +1,8 @@
-<script type="text/javascript" src="<?php echo get_template_directory_uri() ?>/bundle.js"></script>
+<?php
+$js_url = get_template_directory_uri() . '/bundle.js';
+$deploy_timestamp = windycoys_get_deploy_timestamp();
+if ($deploy_timestamp) {
+  $js_url .= '?t=' . $deploy_timestamp;
+}
+?>
+<script type="text/javascript" src="<?php echo $js_url ?>"></script>

theme/functions.php

@@ -1,5 +1,4 @@
 <?php
-
 class Windycoys_Walker_Comment extends Walker_Comment {
   function start_el(&$output, $comment, $depth = 0) {
     $id = $comment->comment_ID;
@@ -84,3 +83,11 @@ function windycoys_search_posts_per_page($query) {
   return $query;
 }
 add_filter('pre_get_posts', 'windycoys_search_posts_per_page');
+
+function windycoys_get_deploy_timestamp() {
+  if (file_exists(__DIR__ . '/.timestamp')) {
+    $timestamp = file_get_contents(__DIR__ . '/.timestamp');
+    return $timestamp;
+  }
+  return false;
+}

theme/header.php

@@ -1,3 +1,10 @@
+<?php
+$css_url = get_template_directory_uri() . '/style.css';
+$deploy_timestamp = windycoys_get_deploy_timestamp();
+if ($deploy_timestamp) {
+  $css_url .= '?t=' . $deploy_timestamp;
+}
+?>
 <!doctype html>
 <html <?php language_attributes(); ?>>
   <head>
@@ -7,7 +14,7 @@
     <meta name="theme-color" content="#4b88b6">
     <meta name="description" content="<?php bloginfo('description'); ?>">
     <?php wp_head(); ?>
-    <link rel="stylesheet" href="<?php echo get_template_directory_uri(); ?>/style.css">
+    <link rel="stylesheet" href="<?php echo $css_url ?>">
     <link href="//fonts.googleapis.com/css?family=Open+Sans|Raleway:700" rel="stylesheet">
   </head>
   <body>