PayPal PHP SDK is our official Open Source PHP SDK for supporting PayPal Rest APIs. Checkout all the supporting documents, samples, codebase from the following links
Find everything from Installing, running Samples, Configurations in PHP SDK Wiki
Check out PHP Source Documentation, to see the internals of PHP SDK.
Check out the sample code for using SDKs to call all PayPal APIs.
Checkout the Official PayPal REST API Reference, explaining all API Models
Download the latest PHP SDK Release