View file application/libraries/Engine/Form/Decorator/Captcha/ReCaptcha2.php

<?php

class Engine_Form_Decorator_Captcha_ReCaptcha2 extends Zend_Form_Decorator_Abstract
{

  /**
   * Render captcha
   *
   * @param  string $content
   * @return string
   */
  public function render($content)
  {
    $element = $this->getElement();
    if( !$element instanceof Zend_Form_Element_Captcha ) {
      return $content;
    }

    $view = $element->getView();
    if( null === $view ) {
      return $content;
    }

    $placement = $this->getPlacement();
    $separator = $this->getSeparator();
    $captcha = $element->getCaptcha();
    $markup = $captcha->render($view, $element);

    switch( $placement ) {
      case 'PREPEND':
        $content = $markup . $separator . $content;
        break;
      case 'APPEND':
      default:
        $content = $content . $separator . $markup;
    }
    return $content;
  }
}