Fix W3 Total Cache W3_Plugin_TotalCache::ob_callback() expected to be a reference

Around a year ago I was playing with W3 Total Cache plugin on HHVM while I got an annoying warning

Warning: Parameter 1 to W3_Plugin_TotalCache::ob_callback() expected to be a reference, value given in /wp-includes/functions.php on line 3269

The funny part is that one of the functions was passing by reference which I still could not find the reason for it, maybe it was a limitation in PHP 4.3. Removing one character “&” fixed the issue so I submitted a pull request to the author although the repository currently does not accept pull request. Later on as PHP 7.0 was released, the issue started to show on PHP as well which brought more attention to this small fix. The users comments included anger such as user @kmob2 who said

this is becoming a running gag

Even more user @pratham2003 proposed a one line bash command to solve the problem

sed -i.bak 's/function ob_callback(&/function ob_callback(/g' /path/to/public_html/wp-content/plugins/w3-total-cache/lib/W3/Plugin/TotalCache.php

I hope that the developers will finally listen to the users and fix it soon!

Warming up WordPress cache, HHVM and testing blog pages

If you are minifying scripts and css files using a caching plugin or using FastCGI cache then you might need to warmup your blog after purging your cache. This is a simple warm up cli script for WordPress to initiate cache or HHVM HHBC and making sure all pages/posts do not have errors. Additionally the script creates a urllist.txt file that you can use with siege to test load your server.

#!/usr/bin/env php
< ?php

if (PHP_SAPI != 'cli') {
    header('Status: 403 Forbidden');
    header('HTTP/1.1 403 Forbidden', true, 403);

define('BASE_PATH', "");
define('WP_USE_THEMES', false);
global $wp, $wp_query, $wp_the_query, $wp_rewrite, $wp_did_header;
require(BASE_PATH . 'wp-load.php');

//Disable cache

$homeUrl = home_url() . "/";
$urlList = [$homeUrl];

$the_query = new WP_Query(['post_type' => 'any', 'post_status' => 'publish']);
//add homepage pages
warmup_pages_links($the_query->max_num_pages, $homeUrl);

//get all posts
$the_query = new WP_Query(['post_type' => 'any', 'posts_per_page' => '-1', 'post_status' => 'publish']);
while ($the_query->have_posts()) {
    $urlList[] = get_permalink();

$categories = get_categories();
foreach ($categories as $category) {
    $catUrl = get_category_link($category->term_id);
    $urlList[] = $catUrl;
    $the_query = new WP_Query(['cat' => $category->term_id]);
    warmup_pages_links($the_query->max_num_pages, $catUrl);

$tags = get_tags();
foreach ($tags as $tag) {
    $tagUrl = get_tag_link($tag->term_id);
    $urlList[] = $tagUrl;
    $the_query = new WP_Query(['tag_id' => $tag->term_id]);
    warmup_pages_links($the_query->max_num_pages, $tagUrl);

$authors = get_users(array('who' => 'authors'));
foreach ($authors as $author) {
    $authorUrl = get_author_posts_url(false, $author->user_nicename);
    $urlList[] = $authorUrl;
    $the_query = new WP_Query(['author_id' => $author->term_id]);
    warmup_pages_links($the_query->max_num_pages, $authorUrl);

//test search page
$urlList[] = get_search_link('bala la lam');

file_put_contents('urllist.txt', implode("\n", $urlList));

echo "Found " . count($urlList) . " URLS and saved to urlList.txt \n";
echo "Testing URLs \n";
foreach ($urlList as $url) {
echo "\nWarmup done \n";

function warmup_getUrl($url)
    if ($contents = wp_remote_get($url, ['timeout' => 15])) {
        if (is_wp_error($contents) || (isset($contents['response']['code']) && $contents['response']['code'] != 200)) {
            $subject = "[WP " . get_bloginfo('name') . "] Error on {$url} ";
            $error = is_wp_error($contents) ? $contents->get_error_message() : print_r($contents, true);
            wp_mail(get_bloginfo('admin_email'), $subject, $error);
            die("\n{$subject} :" . $error);
        echo ".";
    return $contents;

function warmup_pages_links($total, $url)
    for ($i = 2; $i < = $total; $i++) {
        $GLOBALS['urlList'][] = "{$url}page/{$i}/";

// siege -c200 -d1 -r30 -f urllist.txt