Guzzle Promises

Promises/A+ implementation that handles promise chaining and resolution iteratively, allowing for "infinite" promise chaining while keeping the stack size constant. A promise represents the eventual result of an asynchronous operation. The primary way of interacting with a promise is through its then method, which registers callbacks to receive either a promise's eventual value or the reason why the promise cannot be fulfilled. Callbacks are registered with the then method by providing an optional $onFulfilled followed by an optional $onRejected function. Resolving a promise means that you either fulfill a promise with a value or reject a promise with a reason. Resolving a promises triggers callbacks registered with the promises's then method. These callbacks are triggered only once and in the order in which they were added.