use Tylercd100\Monolog\Handler\MailgunHandler;

$to      = "to@test.com";
$from    = "from@test.com";
$subject = "Test Email!";
$token   = "1onfifaln234nfkdo02";
$domain  = "test.com";

$handler = new MailgunHandler($to, $subject, $from, $token, $domain);
$logger  = new Monolog\Logger('mailgun.example');
$logger->pushHandler($handler);
$logger->addCritical("Foo Bar!");