Telling users and machines apart

Hi there, I am getting more and more emails through my webforms submited by bots. It looks like some sites liky yahoo are placing an image into their forms and the human has to enter a code visible on the image to submit the form. After some googling I found captcha, but I do not like the images. Do you know if there is a class available for PHP which does create the image and does parts of the handling? That seems like a common task to me. Maybe someone else already implemented such thing and can recommend. Thank you in advance, Merlin