composer update

composer.json

{
    "name": "root/async-calls",
    "require": {
        "guzzlehttp/guzzle": "~5.2"
    }
}

process.php

<?php

require __DIR__ . '/vendor/autoload.php';

use GuzzleHttp\Client;
use GuzzleHttp\Event\CompleteEvent;
use GuzzleHttp\Pool;

// Run
main();

/**
 * App runner
 *
 * @param int $id                   The incremental start id
 * @param int $chunk                Pool size for making async requests
 * @param string $fAddress          File address to save found values
 * @param string $endPoint          Endpoint which the requests are executed against
 * @param string $incrementalIdName The incremental query string param
 * @param string $lookFor           Strings that match are bowlshit!
 */
function main(
    $id = 50000,
    $chunk = 1,
    $fAddress = 'not.txt',
    $endPoint = 'http://example.com/app/script',
    $incrementalIdName = 'user_id',
    $lookFor = 'Your string'
){
    $client = new Client(); $handle = fopen($fAddress, 'w');

    while(true)
    {
        $requests = [];

        for($i = $id; $i < ($chunk + $id); $i++)
        {
            $request = $client->createRequest(
                'GET',
                $endPoint,
                ['timeout' => 5]
            );

            $request->getQuery()->set($incrementalIdName, $i);

            $requests[] = $request;
        }

        $pool = new Pool($client, $requests, [ 'complete' => function(CompleteEvent $event) use(&$handle, $lookFor)
        {
            $response = (string) $event->getResponse()->getBody();

            if(! contains($response, $lookFor)){

                fwrite($handle, $url = $event->getRequest()->getUrl() . "\n");

                echo "Found : " . ($url) . "\n";

            }else {

                echo "Not found : " . $event->getRequest()->getUrl() . "\n";

            }

        }]);

        $id = $chunk + $id;

        $pool->wait();
    }

    fclose($handle);
}

/**
 * Check that given string contains a set of values or not
 *
 * @param $haystack
 * @param array $needles
 * @return bool
 */
function contains($haystack, $needles)
{
    foreach ((array) $needles as $needle)
    {
        if ($needle != '' && strpos($haystack, $needle) !== false) return true;
    }

    return false;
};