Add this definition to your FeatureContext.php to take a screenshot of failed steps for your Behat/Selenium2 tests. It creates a directory called "screenshots" in your project directory (if one doesn't already exist) and within it places screenshots whose filenames consist of the date and time as well as the name of the failed step.

FailedStepScreenshotDefinition.php

/**
* @AfterStep
*/
public function takeScreenShotAfterFailedStep(afterStepScope $scope)
{
  if (99 === $scope->getTestResult()->getResultCode()) {
    $driver = $this->getSession()->getDriver();

    $filePath = getcwd() . DIRECTORY_SEPARATOR . 'screenshots';

    if (!file_exists($filePath)) {
      mkdir($filePath);
    }

    $stepText = $scope->getStep()->getText();
    $fileTitle = date('Y-m-d--H:i:s') . '--' . preg_replace("#[^a-zA-Z0-9\._-]#", '', $stepText);

    $fileName = $filePath . DIRECTORY_SEPARATOR . $fileTitle . '.png';
    $screenshot = $this->getSession()->getDriver()->getScreenshot();
    file_put_contents($fileName, $screenshot);
    print "Screenshot for '{$stepText}' placed in {$fileName}\n";
  }
}

I worked out how to do this because my tests were working in Chrome and failing in PhantomJS. By looking at the screenshots, I realized that PhantomJS defaults to a very narrow window, which meant that it was testing the mobile site rather than the desktop site (and thus the menus I was testing weren't the same). You can resize your test browser window using this step definition: https://gist.github.com/megclaypool/42dc283beccc8085a15f6e9c106d74f1