Paul Redmond explains how you can use Guzzle 6' middleware to add a HMAC authorization header.

I prefer to keep my dependencies as up-to-date as possible so I decided to learn Guzzle 6 and become more familiar with the middleware. The concepts are pretty straightforward and I have a few patterns that I like to use when building out middleware within my Laravel applications.

https://medium.com/@paulredmond/using-guzzle-6-middleware-in-a-laravel-application-7fbd6d966235