CAPTCHA script for PHP with audio back up for the blind and visually impaired. For the audio it uses a .wav file as this is the most universal. The audio file is telephone quality (6000hz sampling and mono) to keep size down and to cut processing time. The script only uses a 4 character CAPTCHA code but this amounts to some 15 million combinations (26 upper case letters + 26 lower case letters + 10 digits = 62; 624 = 14,776,336). For the paranoid this could easily extended but more characters makes the script more difficult for humans. In the case of machines though they can either crack it or they can't. Extra charcters will make little difference to the cracking abilities of their OCR software. Also having only 4 characters keeps the size of the audio file to a minimum and cuts CPU cycles.
Update: The characters "1", "l", "I", "o", "O", and "0" have been omitted to improve readability.
Download the script including the audio library (275kB .zip file).