librecaptcha

Currently, you need to compile and implement the Provider within a fork of the framework itself. In the future, it will be possible to compile providers separately and install them as a plugin.
For inspiration, see the sample providers. FilterChallenge is simple and a good starting point.
Implement the ChallengeProvider interface, which contains these methods:
- getId() - needs to return a string that will be used to identify the provider itself.
- returnChallenge() - needs to return a new CAPTCHA challenge, for the configured parameters
- checkAnswer() - check whether than answer is correct
- configure() - (optional) use the supplied configuration
- supportedParameters - (optional) a map of parameters and their accepted values