1. Single Responsibility Principle - mrbahadoor/symfony5-solid GitHub Wiki

Definition
A function or class should be responsible for only one task (or responsibility).

Action
Gather together the things that change for the same reason. Separate the things that change for different reasons.
Think of all the different reasons that a class might change but not all the tiny differences.
Examples: Group all emailing function or database / CRUD related function

Example
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