SOLID principles

What it is

SOLID encourages cohesive responsibilities and replaceable collaborators.

When to use it

Use as questions to improve a design, not as a requirement to add abstractions.

Syntax

S: one coherent reason to change
O: extend real variability
L: preserve contract
I: narrow contracts
D: depend on stable boundaries

Example

A payment gateway interface has a concrete benefit when switching providers or testing failures.
A repository wrapping every Eloquent method often adds little benefit in a small app.

Common mistakes

Do not infer that every class needs an interface or that dependency injection requires elaborate layers.

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