A helper class for Codeception that allows automated accessility checks (WCAG 2.0, Section508) using the pa11y command line tool during acceptance testing. It uses local binaries and can therefore be run offline.

AccessibilityValidator.php

<?php

/**
 * A helper class for Codeception (http://codeception.com/) that allows automated accessility checks
 * (WCAG 2.0, Section508) using the pa11y (http://pa11y.org/) command line tool
 * during acceptance testing.
 * It uses local binaries and can therefore be run offline.
 *
 *
 * Requirements:
 * =============
 *
 * - Codeception with WebDriver or PhpBrowser set up
 * - pa11y is installed locally (e.g. using "npm insgall -g pa11y")
 *
 *
 * Installation:
 * =============
 *
 * - Copy this file to _support/Helper/ in the codeception directory
 * - Merge the following configuration to acceptance.suite.yml:
 *
 * modules:
 *   enabled:
 *     - \Helper\AccessibilityValidator
 *   config:
 *     \Helper\AccessibilityValidator:
 *       pa11yPath: /usr/local/bin/pa11y
 *
 *
 * Usage:
 * ======
 *
 * Validate the current site against WCAG 2.0 (AAA):
 * $I->validatePa11y(\Helper\AccessibilityValidator::STANDARD_WCAG2AAA);
 *
 * Validate the current site against WCAG 2.0 (AA):
 * $I->validatePa11y(); // or:
 * $I->validatePa11y(\Helper\AccessibilityValidator::STANDARD_WCAG2A);
 *
 * Validate the current site against WCAG 2.0 (A):
 * $I->validatePa11y(\Helper\AccessibilityValidator::STANDARD_WCAG2A);
 *
 * Validate the current site against Section 508:
 * $I->validatePa11y(\Helper\AccessibilityValidator::STANDARD_SECTION508);
 *
 * Validate against WCAG 2.0 (AA), but ignore errors containing the string "Ignoreme":
 * $I->validatePa11y(\Helper\AccessibilityValidator::STANDARD_WCAG2A, ["Ignoreme"]);
 *
 *
 *
 * @license http://www.opensource.org/licenses/mit-license.html  MIT License
 * @author Tobias Hößl <tobias@hoessl.eu>
 */

namespace Helper;

use Codeception\TestCase;

class AccessibilityValidator extends \Codeception\Module
{
    public static $SUPPORTED_STANDARDS = array(
        'WCAG2AAA',
        'WCAG2AA',
        'WCAG2A',
        'Section508',
    );
    const STANDARD_WCAG2AAA   = 'WCAG2AAA';
    const STANDARD_WCAG2AA    = 'WCAG2AA';
    const STANDARD_WCAG2A     = 'WCAG2A';
    const STANDARD_SECTION508 = 'Section508';

    /**
     * @return string
     */
    private function getPageUrl()
    {
        if ($this->hasModule('WebDriver')) {
            /** @var \Codeception\Module\WebDriver $webdriver */
            $webdriver = $this->getModule('WebDriver');
            return $webdriver->webDriver->getCurrentURL();
        } else {
            /** @var \Codeception\Module\PhpBrowser $phpBrowser */
            $phpBrowser = $this->getModule('PhpBrowser');
            return trim($phpBrowser->_getUrl(), '/') . $phpBrowser->_getCurrentUri();
        }
    }

    /**
     * @param string $url
     * @param string $standard
     * @return array
     * @throws \Exception
     */
    private function validateByPa11y($url, $standard)
    {
        if (!in_array($standard, static::$SUPPORTED_STANDARDS)) {
            throw new \Exception('Unknown standard: ' . $standard);
        }

        $pa11yPath = $this->_getConfig('pa11yPath');
        if (!$pa11yPath) {
            $pa11yPath = 'pa11y';
        }
        if (!file_exists($pa11yPath)) {
            throw new \Exception('pa11y not found: ' . $pa11yPath);
        }

        exec($pa11yPath . " -s " . $standard . " -r json '" . addslashes($url) . "'", $return);
        $data = json_decode($return[0], true);
        if (!$data) {
            $msg = 'Invalid data returned from validation service: ';
            throw new \Exception($msg . $return);
        }
        return $data;
    }

    /**
     * @param string $standard
     * @param string[] $ignoreMessages
     */
    public function validatePa11y($standard = 'WCAG2AA', $ignoreMessages = [])
    {
        try {
            $url      = $this->getPageUrl();
            $messages = $this->validateByPa11y($url, $standard);
        } catch (\Exception $e) {
            $this->fail($e->getMessage());
            return;
        }
        $failMessages = [];
        foreach ($messages as $message) {
            if ($message['type'] == 'error') {
                $string = $message['code'] . "\n" . $message['selector'] . ': ';
                $string .= $message['context'] . "\n";
                $string .= $message['message'];
                $ignoring = false;
                foreach ($ignoreMessages as $ignoreMessage) {
                    if (mb_stripos($string, $ignoreMessage) !== false) {
                        $ignoring = true;
                    }
                }
                if (!$ignoring) {
                    $failMessages[] = $string;
                }
            }
        }
        if (count($failMessages) > 0) {
            $failStr = 'Failed ' . $standard . ' check: ' . "\n";
            $failStr .= implode("\n\n", $failMessages);
            \PHPUnit_Framework_Assert::fail($failStr);
        }
    }
}

If anyone is having trouble getting the yaml file set up this is how I have mines setup and is working for me. For the pa11yPath make sure you know where your path is this helped me out I used in the terminal 'which pa11y' to see the absolute file path.
actor: AcceptanceTester
modules:
enabled:
- WebDriver:
url: http://example.com
browser: phantomjs
window_size: 1280x960
restart: true
- \Helper\Acceptance
- \Helper\AccessibilityValidator
- \Helper\AccessibilityValidator:
config:
pa11yPath: tests/pa11y/