DHL PHP SDK is an unofficial resource that wraps some of the DHL SOAP API's functions, including shipment and label creation. Even though the SDK is unofficial, developers still need a DHL developer account (and possibly a DHL Intraship Account) to use the API.
DropBuddies, the Postmates of Nigeria, launched an API that allows local businesses to include courier services directly within their apps. Without leaving the integrated business' app, users get a delivery quote, ETA, and book a delivery. Once booked, users can track shipments through delivery.
Adding shipping and posting functionalities to your applications may be necessary for your what you're trying to achieve, but it may not always be a quick and easy process. EasyPost is a posting and shipping service specifically designed for applications, saving developers some very valuable development time. The EasyPost API makes it possible for developers to access this functionality and easily integrate it with other applications.
It's not enough to print stamps off your printer any more. What shippers really want is what the SOAP-based Stamps.com API does: integrate that function of printing postage into your own web-based ecommerce or web hosted applications to speed up the shipment of orders out the door. With the API you can do much more than calculate postage and print stamps. You can clean addresses and track packages, among other tasks.