PHP script not working to replace WordPress cron for multi-site

Question

I am trying to replace the WordPress cron with the server cron for my multi-site. It seems that I have to add a cron entry for EACH site which could be a management nightmare. Instead, I want to run a single cron job that calls the cron for each sub-site. Below is a plugin script, but getting error 500. The error message is misleading and incorrect because it says something about the ffmpeg not installed (no other error message). Can someone please help me correct this? The plugin is in GitHub (https://github.com/chibani/wp_multisite_cron) but I think would make a great plugin for the community. Thanks for any help!

register_activation_hook(__FILE__, array('wp_multisite_cron','plugin_activation'));
add_action('init', array('wp_multisite_cron', 'init'));

class wp_multisite_cron{

    const LANG = 'wp_multisite_cron';
    const LANG_DIR = '/lang/';

    /**
     * 
     * Plugin activation (sets default parameters)
     */
    public static function plugin_activation(){
        if(!self::get_option('concurrent_crons'))
            self::update_option('concurrent_crons', 100);
    }

    /**
     * 
     * The main 'loader'
     */
    public static function init() {

        //Setup the translation
        load_plugin_textdomain(self::LANG, false, dirname(plugin_basename( __FILE__ ) ) . self::LANG_DIR);

        //The multisite cron action     
        add_action('wp_ajax_wp_multisite_cron_call', array('wp_multisite_cron','cron_call'));
        add_action('wp_ajax_nopriv_wp_multisite_cron_call', array('wp_multisite_cron','cron_call'));

        // admin actions and hooks
        if (is_admin()) {
            self::admin_hooks();
        }
    }

    /**
     * 
     * The admin hooks
     */
    public static function admin_hooks(){
        //Setting menu
        add_action('admin_menu', array('wp_multisite_cron', 'admin_menu'));
        if(is_network_admin() && self::is_ready()){
            add_action('network_admin_menu', array('wp_multisite_cron', 'network_admin_menu'));
        }
    }

    /**
     * 
     * Set up the admin menu(s)
     */
    public static function admin_menu(){
        add_options_page("WordPress multisite cron, information page", "WP multisite cron", 'manage_options', 'wp_multisite_cron_settings', array('wp_multisite_cron', "admin_settings"));
    }

    /**
     * 
     * Set up the admin menu(s)
     */
    public static function network_admin_menu(){
        add_submenu_page("settings.php","WordPress multisite cron, information page", "WP multisite cron", 'manage_options', 'wp_multisite_cron_settings', array('wp_multisite_cron', "admin_settings"));
    }


    /**
     * 
     * The admin settings page
     */
    public static function admin_settings(){
        ?>
        <div class="wrap">
            <div id="icon-options-general" class="icon32">
                <br />
            </div>
            <h2>WordPress multisite cron</h2>

            <?php if (isset($_POST['wp_multisite_cron']) && !empty($_POST['wp_multisite_cron'])) :

                if (isset($_POST['_wpnonce']) && wp_verify_nonce( $_POST['_wpnonce'], plugin_basename( __FILE__ ) )) :
                    self::delete_option('concurrent_crons');

                    foreach($_POST['wp_multisite_cron'] as $option_name=>$option_value){
                        self::update_option($option_name, $option_value);
                    }
                ?>
                    <div id="setting-error-settings_updated" class="updated settings-error">
                        <p>
                            <strong><?php _e('Settings saved.')?></strong>
                        </p>
                    </div>
                <?php else: ?>
                    <div id="message" class="error fade">
                        <p><?php _e('Unable to update settings.',self::LANG)?></p>
                    </div>
                <?php endif;?>
            <?php endif;?>



            <?php if(!is_multisite()):?>
                <div class="tool-box error">
                    <h3><?php _e('Multisite',self::LANG) ?></h3>
                    <p><?php _e('Your WordPress installation is not configured as multisite',self::LANG) ?>.</p>
                    <p><?php _e("Read the WordPress' documentation about multisite to learn how to set up multisite",self::LANG)?> : <a href="http://codex.wordpress.org/Create_A_Network" target="_blank"><?php _e('here',self::LANG)?></a></p>
                </div>
            <?php endif;?>


            <?php if(!defined('DISABLE_WP_CRON') || !DISABLE_WP_CRON ):?>
                <div class="tool-box error">
                    <h3><?php _e('WordPress default cron system',self::LANG) ?></h3>
                    <p><?php _e('You must disable the WordPress cron system, to use this extension',self::LANG) ?>.</p>
                    <p><?php _e('Add the following code in your wp-config.php',self::LANG) ?> :</p>
                    <p>
                        <code>define('DISABLE_WP_CRON', true);</code>
                    </p>
                </div>
            <?php endif;?>

            <form action="" method="post">
                <table class="form-table">
                    <tbody>
                        <tr valign="top">
                            <th scope="row">
                                <label for="concurrent_crons"><?php _e('Concurrent crons',self::LANG) ?></label><br />
                                <em><?php _e("Lower it if some of your sites' cron don't run",self::LANG)?></em>
                            </th>
                            <td>
                                <input type="text" name="wp_multisite_cron[concurrent_crons]" id="concurrent_crons" value="<?php echo self::get_option('concurrent_crons') ?>" />
                            </td>
                        </tr>
                    </tbody>
                </table>
                <p class="submit">
                    <input class="button-primary" name="plugin_ok" value="<?php _e('Save settings',self::LANG) ?>" type="submit" />
                </p>
                <?php wp_nonce_field( plugin_basename( __FILE__ ), '_wpnonce' );?>
            </form>


            <?php if(self::is_ready()): ?>
                <div class="tool-box">
                    <h3><?php _e('Set-up WordPress multisite cron',self::LANG) ?></h3>
                    <p><?php _e('Add the following line in your crontab',self::LANG)?> :</p>
                    <p>
                        <code>*/15 *<?php echo "\t" ?>* * * www-data /usr/bin/wget -qO- <?php echo admin_url('admin-ajax.php','http').'?action=wp_multisite_cron_call' ?></code>
                    </p>
                </div>
            <?php endif;?>
        </div>

        <?php 
    }

    /**
     * 
     * Check wether the config is ok to run real crons
     */
    public static function is_ready(){
        return (is_multisite() && defined('DISABLE_WP_CRON') && DISABLE_WP_CRON);
    }

    /**
     * 
     * The real cron
     */
    public static function cron_call(){

        //load the blogs id
        $blogs = self::get_blogs_id();
        if(count($blogs)){
            $mh = curl_multi_init();

            $length = self::get_option('concurrent_crons');
            $offset = 0;

            //Cut the 
            while($blogs_slice = array_slice($blogs,$offset,$length)){

                $offset += $length;

                foreach($blogs_slice as &$blog_id){
                    switch_to_blog($blog_id);
                    $cron_url = site_url().'/wp-cron.php?doing_wp_cron';

                    //Add the url to the stack
                    $chs[$blog_id]=curl_init();
                    curl_setopt($chs[$blog_id], CURLOPT_URL, $cron_url);
                    curl_setopt($chs[$blog_id], CURLOPT_HEADER, 0);
                    curl_multi_add_handle($mh, $ch);
                }

                //Launch :)
                $running = null;
                do{
                    curl_multi_exec($mh, $still_running);
                }while($still_running > 0);

                curl_multi_close($mh);
            }
        }
        die();
    }

    /**
     * 
     * Get the blog_id for each active blog in the multisite
     * @return array
     */
    public static function get_blogs_id(){
        global $wpdb;

        //Get the blogs' ids for blogs that are public and active
        $blogs = $wpdb->get_col('SELECT blog_id FROM '.$wpdb->blogs.' WHERE public=1 AND deleted=0');
        return $blogs;
    }

    /**
     * 
     * Get a plugin's specific option
     * @param string $option_name
     */
    public static function get_option($option_name){
        return get_option('wp_multisite_cron_'.$option_name);
    }

    /**
     * 
     * Set a plugin's specific option
     * @param unknown_type $option_name
     */
    public static function update_option($option_name,$option_value){
        return update_option('wp_multisite_cron_'.$option_name,$option_value);
    }

    /**
     * 
     * Delete a plugin's specific option
     * @param string $option_name
     */
    public static function delete_option($option_name){
        return delete_option('wp_multisite_cron_'.$option_name);
    }


    /**
     * 
     * get the plugin's path url
     */
    public static function get_plugin_url(){
        return get_bloginfo('url') . '/' . PLUGINDIR . '/' . dirname(plugin_basename(__FILE__));
    }
}

Answer 1

Within your cron_call function, around line 200, you have this block:

//Add the url to the stack
$chs[$blog_id]=curl_init();
curl_setopt($chs[$blog_id], CURLOPT_URL, $cron_url);
curl_setopt($chs[$blog_id], CURLOPT_HEADER, 0);
curl_multi_add_handle($mh, $ch);

So first you initialize your curl handlers in the $chs array. Then, instead of using those values, you try to add the $ch variable to your multi handler, which is null I presume.

With proper error reporting settings and by checking the result of curl_multi_add_handle, you could probably see that the add handle operation failed.

From the php manual:

int curl_multi_add_handle ( resource $mh , resource $ch )

Return Values

Returns 0 on success, or one of the CURLM_XXX errors code.

EDIT: maybe I was not clear enough on the solution, but you need to replace

curl_multi_add_handle($mh, $ch);

with

curl_multi_add_handle($mh, $chs[$blog_id]);