This TextKey PHP library allows a developer to implement REST API calls server-side from a PHP backend. TextKey is an omni-factor authentication API that uses seven-factors of authentication to provide security for devices ranging from mobile to electronic door locks. TextKey works through using SMS service, unique code, cellular number, UDID, a single use URL, and an optional pin. TextKey uses REST with a JSON response and SOAP. TextKey is free for ten users, and available for a monthly charge per user thereafter.
Yahoo! has announced the rollout of some limited tests for OpenID's Simple Registration specification. If you're not familiar with OpenID, it's an innovative way for handling user authentication that provides a free and easy way to use a single digital identity across the Internet.