example.php

<?php
namespace Facebook\WebDriver;

use Facebook\WebDriver\Chrome\ChromeOptions;
use Facebook\WebDriver\Remote\DesiredCapabilities;
use Facebook\WebDriver\Remote\RemoteWebDriver;

require_once('vendor/autoload.php');

// start Chrome with 5 second timeout
$host = 'http://localhost:4444/wd/hub'; // this is the default
$capabilities = DesiredCapabilities::chrome();


$options = new ChromeOptions();

$options->addArguments(array(
    "--headless", "window-size=1024,768", "--no-sandbox"
));
$prefs = array(
    'permissions.default.stylesheet' => 2,
    'permissions.default.image' => 2
);
$options->setExperimentalOption('prefs', $prefs);
$capabilities->setCapability(ChromeOptions::CAPABILITY, $options);

$driver = RemoteWebDriver::create($host, $capabilities, 5000);

$driver->get('http://localhost/test/');

function isAlertPresent($driver){
    try{
        $driver->wait(10)->until(WebDriverExpectedCondition::alertIsPresent());
        $driver->switchTo()->alert()->accept();

        return true;
    } catch (Exception $ex) {
        return false;
    }
}

$alertFound = isAlertPresent($driver);
// close the browser
$driver->quit();
var_dump($alertFound);