SOLID in PHP

 https://dev.to/evrtrabajo/solid-in-php-d8e

good overview of how SOLID principles work in PHP

SOLID Principles every Developer Should Know

https://blog.bitsrc.io/solid-principles-every-developer-should-know-b3bfa96bb688
Good examples of SOLID principles. Some of these are fairly painful.

An introduction to SOLID, Tim Berners-Lee’s new, re-decentralized Web

https://medium.freecodecamp.org/an-introduction-to-solid-tim-berners-lees-new-re-decentralized-web-25d6b78c523b
A slightly more cogent explanation of Tim B-Ls SOLID.

When to add an interface to a class

https://matthiasnoback.nl/2018/08/when-to-add-an-interface-to-a-class/
A lot of people (myself included) are fuzzy on this concept. Good SOLID article. 

8 resources to learn about SOLID design principles

https://barryvanveen.nl/blog/51-8-resources-to-learn-about-solid-design-principles
SOLID design principle resources.

SOLID Principles made easy

https://dev.to/dhurimkelmendi/solid-principles-made-easy-1pg
the classic SOLID principles explained.