Maxim Mironjuk
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April 15, 2025
Most explanations of SOLID principles work with shapes, ducks, or animals, and leave open what that looks like in real application code. This article instead uses a single, connected order-processing pipeline in PHP 8.4, in which each of the five principles solves a concrete, traceable maintainability problem: an Order class that does too much, discount rules that grow into an if/else cascade, a payment gateway that breaks its own contract, an overly fat repository interface, and a notification service too tightly coupled to a concrete mailer.