Mailtrap, Zend Framework 2, and PhpUnit test case

MailtrapTestCase.php

<?php

use InvalidArgumentException;
use PHPUnit_Framework_TestCase as TestCase;
use Zend\Mail\Transport\Smtp as SmtpTransport;
use Zend\Mail\Transport\SmtpOptions;

/**
 * A base testcase for mail transactions with Mailtrap.
 *
 * The following constants must be specified in the phpunit.xml config file:
 * - MAILTRAP_TOKEN
 * - MAILTRAP_INBOX_NAME
 * - MAILTRAP_INBOX_PASSWORD
 *
 * @see http://mailtrap.io
 */
abstract class MailtrapTestCase extends TestCase
{
    const MAILTRAP_API_URL = 'http://mailtrap.io/api/v1/inboxes';
    const MAILTRAP_AUTH    = 'plain';
    const MAILTRAP_HOST    = 'mailtrap.io';
    const MAILTRAP_PORT    = 2525;

    protected static $smtpOptions = [
        'host'              => self::MAILTRAP_HOST,
        'port'              => self::MAILTRAP_PORT,
        'connection_class'  => self::MAILTRAP_AUTH,
        'connection_config' => [
            // from phpunit.xml
            'username' => MAILTRAP_INBOX_NAME,
            'password' => MAILTRAP_INBOX_PASSWORD,
        ]
    ];

    protected function getMailTransport()
    {
        return new SmtpTransport(new SmtpOptions(static::$smtpOptions));
    }

    protected function getInboxesEndpoint()
    {
        return $this->buildApiEndpoint();
    }

    protected function getMessagesEndpoint($page = 1, $inbox = MAILTRAP_INBOX_NAME)
    {
        $this->assertString($inbox, "Inbox must be a string");
        return $this->buildApiEndpoint("/$inbox/messages?page=$page");
    }

    protected function getMessageEndpoint($messageId, $inbox = MAILTRAP_INBOX_NAME)
    {
        $this->assertString($inbox, "Inbox must be a string");
        $this->assertString($messageId, "Message id must be a string");
        return $this->buildApiEndpoint("/$inbox/messages/$messageId");
    }

    private function buildApiEndpoint($path = '')
    {
        if (strpos($path, '?') === false) {
            $path .= '?';
        } else {
            $path .= '&';
        }
        return static::MAILTRAP_API_URL . "{$path}token=" . MAILTRAP_TOKEN;
    }

    private function assertString($string, $message = null)
    {
        if (!is_string($string)) {
            throw new InvalidArgumentException($message);
        }
    }
}