Take a screenshot of a specific part (CSS selector) in Behat

foo.php

  <?php
    
    /**
     * @Then /^take a screenshot of "([^"]*)" and save "([^"]*)"$/
     */
    public function takeAScreenshotOfAndSave($selector, $filename)
    {
        $this->saveScreenshot($filename, '/tmp');
        $pos = $this->getSession()->evaluateScript('$$("' . $selector . '").first().getBoundingClientRect();');

        $dst_image = imagecreatetruecolor(round($pos['width']), round($pos['height']));
        $src_image = imagecreatefrompng("/tmp/" . $filename);
        imagecopyresampled($dst_image, $src_image,
            0, 0,
            round($pos['left']), round($pos['top']), round($pos['width']), round($pos['height']),
            round($pos['width']), round($pos['height']));
        imagepng($dst_image, "/tmp/" . $filename);
    }