Curl mutli threading: How to stop the children from timing out when each child has a big batch of data?

Question

we have 60k customers we need to renew their subscription, for that, we use the renewal script which uses curl_multi_exec to create children to call another script which do the transactions and insert is_paid=1 in the database.

The script runs using a cron job, on PHP 5.3 and Apache, centos 6 server. The script always take between 30 to 50 minutes to finish. The script below creates children which run another script which call an API using curl and the API calls a payment gateway using curl and update the database.

If $number_of_items = 400; Of something like that, all the curl request will be done but the cpu will be so high that MySQL will stop inserting records in the database. So you pay but is_paid=0, MySQL won't update it, so around 2% of the users won't have access to the content they paid for

If the number_of_items=6000, it means fewer curl threads, and more number of items per thread, cpu is great, no problem at all, but not all people get their subscription renewed. if we have 60k customers, 10 curl threads get created successfully but maybe 1 finish half its job, we end up with 8k customers renewed or something like that, so they don't even call the payment API

<?php
ini_set('max_execution_time', 0);
ini_set('memory_limit', '-1');
ini_set('display_errors', 'Off');
ini_set("default_socket_timeout", -1);
$filename = __DIR__ . "/subscriptionRenewalAPI.lock";
$lifelimit = 86400; // in Second lifetime to prevent errors
/* check lifetime of file if exist */
if (file_exists($filename)) {
    $lifetime = time() - filemtime($filename);
} else {
    $lifetime = 0;
}

/* check if file exist or if file is too old */
if (!file_exists($filename) || $lifetime > $lifelimit) {
    if ($lifetime > $lifelimit) {
        unlink($filename); //Suppress if exist and too old
    }

    $file = fopen($filename, "w+"); // Create lockfile

    if ($file == false) {
        die("file didn't create, check permissions");
    }

    //Fetch orders that expires and needs renewal TODAY
    //=========================================================================Fetch orders that needs renewal (getOrdersForRenewal) ====================================\\
    // create a new cURL resource
    $ch = array();
    $ch2 = curl_init();
    $mh = curl_multi_init();


    // set URL and other appropriate options
    curl_setopt($ch2, CURLOPT_URL, "http://localhost/crudAPI_mobile/getOrdersForRenewal?auth=staticAuthApli");
    curl_setopt($ch2, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($ch2, CURLOPT_HEADER, 0);
    if ($response = curl_exec($ch2)) {

        if ($response != -1) {
            $orders_for_renewal = json_decode($response, true);


            $begining = 0;
            $number_of_items = 400;

            $curl_counter = 0;

            while ($begining < sizeof($orders_for_renewal)) {  //For each batch -> open a new curl

                $output = array_slice($orders_for_renewal, $begining, $number_of_items);


                //Open a new curl channel
                $ch[$curl_counter] = curl_init();

                //Time to iterate and renew
                $data = array('output' => $output);

                $data = http_build_query($data);
                curl_setopt($ch[$curl_counter], CURLOPT_URL, "http://localhost/crons/subscriptionRenewalAPIExecution.php");
                curl_setopt($ch[$curl_counter], CURLOPT_POST, true);
                curl_setopt($ch[$curl_counter], CURLOPT_POSTFIELDS, $data);
                curl_setopt($ch[$curl_counter], CURLOPT_HEADER, 0);
                curl_setopt($ch[$curl_counter], CURLOPT_RETURNTRANSFER, true);
                curl_multi_add_handle($mh, $ch[$curl_counter]); //Add it to the multi curl handler

                $begining += $number_of_items;
                $curl_counter++;
            } //end of while batch

            $renewed_orders = 0;
            $churned_orders = 0;

            //Execute the conccurent curl calls prepared
            $active = null;
            $do_while_counter = 0;
            do {

                $mrc = curl_multi_exec($mh, $active);
                curl_multi_select($mh);
                $do_while_counter++;
                sleep(1); // Maybe needed to limit CPU load (See P.S.)
            } while ($active);
            $content = array();
            $i = 0;
            $number_of_curl_requests = 0;
            foreach ($ch as $i => $c) {
                $content[$i] = curl_multi_getcontent($c);
                $content[$i] = unserialize($content[$i]);
                $number_of_curl_requests++;

                curl_multi_remove_handle($mh, $c);
            }
        } 

    }
    // close cURL resource, and free up system resources
    curl_multi_close($mh);
    curl_close($ch2);
    unlink($filename); //Suppress lock file after your process
} else {
    exit(); // Process already in progress
}

I want to keep a relatively big batch of data to ensure that the CPU won't be crazy, and I want to ensure that each child isn't timing out.