Take screenshot with Behat/Mink after failed step

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/**
 * Take screenshot when step fails.
 * Works only with Selenium2Driver.
 * 
 * @AfterStep
 */
public function takeScreenshotAfterFailedStep($event)
{
    if (4 === $event->getResult()) {
        $driver = $this->getSession()->getDriver();
        if (!($driver instanceof Selenium2Driver)) {
            //throw new UnsupportedDriverActionException('Taking screenshots is not supported by %s, use Selenium2Driver instead.', $driver);
            return;
        }

        $screenshot = $driver->wdSession->screenshot();
        file_put_contents('/tmp/test.png', base64_decode($screenshot));
    }
}

working with behat 3.0:

use Behat\Mink\Driver\Selenium2Driver;
use Behat\Behat\Hook\Scope\AfterStepScope;

  /**
   * @AfterStep
   */
  public function takeScreenShotAfterFailedStep(afterStepScope $scope)
  {
    if (99 === $scope->getTestResult()->getResultCode()) {
      $driver = $this->getSession()->getDriver();
      if (!($driver instanceof Selenium2Driver)) {
        return;
      }
      file_put_contents('/tmp/test.png', $this->getSession()->getDriver()->getScreenshot());
    }
  }

Note, error code is changed to 99 and the getScreenshot method now returns a binary file, rather than a base64 encoded string.

This is also automatically supported by our behat 3.x extension, amongst other useful features.
https://github.com/devinci-code/devinci-behat-extension

There is a dedicated Behat extension (bex/behat-screenshot) for taking screenshots, which can be installed via Composer easily.

It can upload the created image to public image hosting sites and display the URL to it on the console. If you need to upload images to a private server, you can create an image adapter for that easily.
Screenshot taking mode allows to make a combined image of previous steps, which can come handy if one of the previous step is responsible for the failing test case.

If you interested, have a look here:
https://github.com/elvetemedve/behat-screenshot

Another dedicated extension with support for Mink (Goutte) and Selenium screenshots
https://github.com/integratedexperts/behat-screenshot

Another extension that gives more than just a screenshot with minimal setup! https://github.com/forceedge01/behat-fail-aid