Caching heavy processing for 24 hours in WordPress

caching-result-of-heavy-processing.php

<?php 
/**
 * This code helps you cache a chunk of heavy processing code in WordPress. 
 * 
 * Examples: 
 *	- WooCommerce has quite a lot of these in their plugins. I've used 
 *	  this general concept on WooCommerce sites many times, but when using
 * 	  it you must be strategic.
 *	- Membership plugins or anything behind authentication busts the caching layer, 
 *	  because the content needs to be pulled from the server. 
 * 
 * The idea is to let the processing happen once, grab the result of the processed php, then cache 
 * it for some time (12 hours in this example) so all visitors in the 12 hour timeframe are only 
 * waiting on the HTML instead of the processing of MySQL and PHP, then the resulting HTML. 
 * 
 * This could introduce side affects if any plugins queue up assets inside of its first function call. 
 * (that is their fault though, and is very, very rare)
 * 
 * Do not cache anything that is: 
 *   1. Updated based for a particular user (like Carts, or anything that may have "Added to Cart" in the page. )
 *   2. Updated based on quantity (If you have a "quantity remaining" count on a product)
 *   3. Updated for different users. (This includes Advertisements or some types of Analytics)
 * 
 * Uses these functions, all available on Flywheel:
 * 
 * Buffers:
 * 	`ob_start`, Docs: http://php.net/manual/en/function.ob-start.php
 * 	`ob_get_clean`, Docs: http://php.net/manual/en/function.ob-get-clean.php
 * 
 * WordPress Transients API: 
 * 	`get_transient`, Docs: https://codex.wordpress.org/Function_Reference/get_transient
 *	`set_transient`, Docs: https://codex.wordpress.org/Function_Reference/set_transient
 * 
 */

$content_transient = false;
$content_transient = get_transient("key_for_this_content"); // You may need to anme this the slug of the current page, and the section of the page you are caching. 

// Forces you to use live content if WP_DEBUG is on.
if (!$content_transient || WP_DEBUG) {
	// Opens a buffer to capture HTML.
	ob_start();
		// This is the block of stuff that has heavy processing. Ideally it's not dynamic, in that it changes every page load.
		
		// Here's a small example
		foreach (array_fill(0, 100, 'banana') as $key => $item):
			echo "<p>" . $item . "</p>";
		endforeach;

	// Puts the result of everything above this line into $content;
	$content = ob_get_clean();

	// This will cache this generated HTML for 12 hours. Whenever someone asks for this HTML, it will pull directly from MySQL or redis. 
	set_transient( "key_for_this_content", $content, 12 * HOUR_IN_SECONDS);
else {
	// set the cached data in the database
	$content = $content_transient;
}

// Simply echo the HTML.
echo $content;