Display Captcha Image on Opencart Product Page
Just change your function getcode() in captcha.php in system library file and you will get captcha image on product review form in opencart. If,still you are not getting captcha just delete your browser cache and cookies and you will get it successfully.
It will display captcha image in product review form.
$out = ob_get_contents();
$out = str_replace(array("\n", "\r", "\t", " "), "", $this->code);