PHP OAuth API authorizes user access using the OAuth protocol. It abstracts OAuth 1.0, 1.0a, and 2.0 in the same class. It provides built-in support for popular OAuth servers: 37Signals, Amazon, Buffer, Bitbucket, Box.net, Discogs, Disqus, Dropbox with OAuth 1.0 and 2.0, Etsy, Eventful, Facebook, Fitbit, Flickr, Foursquare, github, Google with OAuth 1.0a and OAuth 2.0, Instagram, LinkedIn, Microsoft, Rdio, Reddit, RightSignature, Salesforce, Scoop.it, StockTwits, SurveyMonkey, Tumblr, Twitter, VK, Withings, Xero, XING, and Yahoo!. Every other OAuth server is supported by setting an endpoint URLs and other parameters using specific class variables or an external JSON configuration file. The class can also send requests to an API using the previously-obtained OAuth access token. For servers which support offline access, the class can renew expired tokens automatically using refresh tokens.