Simple CAPTCHA for humans.
It's easy. Create a folder in the directory 'images'. Put into 3 files with images in the format .jpg, whose names are 1, 2, 3 (like 1.jpg, 2.jpg, 3.jpg). The picture with the correct answer must be named 1.jpg. Create a style sheet style.css file and issue text.xx.txt, where xx - language (en - English, ru - Russian).