wp-recache/includes/Cache/WPCache.php

<?php
/**
 * Core page cache functionality.
 *
 * @package WP_Recache
 */

defined('ABSPATH') || exit;

class WP_Recache_WPCache {

    /**
     * Initialize cache hooks.
     */
    public function init() {
        add_action('init', [$this, 'start_output_buffering']);
        add_action('shutdown', [$this, 'end_output_buffering']);

        add_action('save_post', [$this, 'purge_post_cache']);
        add_action('wp_insert_comment', [$this, 'purge_post_cache']);

        add_filter('heartbeat_received', [$this, 'purge_cache_onheartbeat']);
    }

    /**
     * Start output buffering.
     */
    public function start_output_buffering() {
        if (wp_recache_should_exclude()) {
            return;
        }

        if (wp_recache_is_cache_enabled()) {
            $this->serve_cached_page();
        }
    }

    /**
     * Serve cached page if available.
     */
    private function serve_cached_page() {
        $url = $this->get_current_url();
        $cache_path = wp_recache_get_cache_path_with_device($url);

        if (file_exists($cache_path)) {
            $modified_time = filemtime($cache_path);
            $cache_duration = wp_recache_get_option('cache_duration', 3600);

            if ((time() - $modified_time) < $cache_duration) {
                header('X-WP-Recache: HIT');
                header('X-WP-Recache-Time: ' . date('Y-m-d H:i:s', $modified_time));
                readfile($cache_path);
                exit;
            }
        }

        header('X-WP-Recache: MISS');
        ob_start([$this, 'cache_output']);
    }

    /**
     * Cache the output.
     *
     * @param string $output Page output.
     * @return string Original output.
     */
    public function cache_output($output) {
        if (empty($output)) {
            return $output;
        }

        $url = $this->get_current_url();
        $cache_path = wp_recache_get_cache_path_with_device($url);
        $cache_dir = dirname($cache_path);

        if (!is_dir($cache_dir)) {
            wp_mkdir_p($cache_dir);
        }

        file_put_contents($cache_path, $output);

        return $output;
    }

    /**
     * End output buffering.
     */
    public function end_output_buffering() {
        if (ob_get_level()) {
            ob_end_flush();
        }
    }

    /**
     * Purge cache for a post.
     *
     * @param int $post_id Post ID.
     */
    public function purge_post_cache($post_id) {
        $url = get_permalink($post_id);

        if ($url) {
            wp_recache_delete_cache($url);
        }

        wp_recache_delete_cache(home_url('/'));
    }

    /**
     * Purge cache on heartbeat.
     *
     * @param array $response Heartbeat response.
     * @return array Modified response.
     */
    public function purge_cache_onheartbeat($response) {
        if (isset($response['wp_recache_purge'])) {
            wp_recache_delete_all_cache();
        }

        return $response;
    }

    /**
     * Get current request URL.
     *
     * @return string Current URL.
     */
    private function get_current_url() {
        $protocol = is_ssl() ? 'https' : 'http';
        $host = $_SERVER['HTTP_HOST'];
        $uri = $_SERVER['REQUEST_URI'];

        return $protocol . '://' . $host . $uri;
    }
}