Is there a free PHP library that can be used for captcha

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In the our website, we adopt reCAPTCHA to prevent spam. However, our clients complain that the validation is too complicated. I also agree that the reCAPTCHA is way too complicated for a regular person to read. It is especially hard for people who don't know English.I found that the CAPTCHA function of mail.yahoo.com is reasonable and I don't know whether or not I can use it for free like reCAPTCHA.Thank youUpdateI think my original idea is to find a free PHP library that can be used for captcha. I just need some simple way to do the captcha rather than make my clients feel it is so difficult even for a real human to solve the words.