Via this digg.com post today, there's information about a new PECL extension that's been committed allowing developers to create wave forms for strings of text.

Yahoo developer Jeremy Johnstone just added a new extension to the PECL PHP library that will allow you to send a string of text and have the extensions spit back out a wave form. This is huge news for those running public sites with the "please enter the text you see here" dialogs. Now you can offer a new alternative to image captcha.

According to its PECL page, it wraps around the Flite voice synthesis library, and can take in text and quickly generate both RIFF and raw wav formats. For more info, check out his new blog entry.