SOLID Principles
S Single Responsibility

SOLID — 01

Single Responsibility

One class — one reason to change

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SOLID — 01 / Flow

S

Single Responsibility

A class should have one, and only one, reason to change.

UserManager
BAD — validates + saves + emails + reports
↓ Split by SRP
UserValidator
OK — validates only
UserRepository
OK — saves only
EmailService
OK — emails only
ReportGenerator
OK — reports only

What you’ll learn

Clean CodeModular PluginsScalable Themes

Page sections

01 Core Concept 02 WordPress Examples 03 Real-World Case Study 04 Common Mistakes 05 Warning Signs 06 FAQ 07 Official Resources
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O Open/Closed

SOLID — 02

Open/Closed

Open for extension, closed for modification

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SOLID — 02 / Flow

O

Open/Closed

Add new features without changing existing code.

Core Plugin
CLOSED — never modified
do_action(“before_save”) — Hook = Extension Point
Your Plugin
OK — add_action() extends without touching core
Another Plugin
OK — add_filter() extends independently

What you’ll learn

Open for ExtensionClosed for ModificationPlugin Architecture

Page sections

01 Core Concept 02 WordPress Hooks 03 Payment Gateways 04 Theme Extensions 05 WooCommerce Example 06 Common Violations 07 FAQ
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L Liskov Substitution

SOLID — 03

Liskov Substitution

Subtypes must be substitutable for base types

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SOLID — 03 / Flow

L

Liskov Substitution

If B extends A — B must work wherever A is expected.

Shape (interface)
getArea(): float
Rectangle implements Shape
OK — width × height
Square implements Shape
OK — side²
printArea(Shape $shape)
OK — works with ANY Shape — LSP

What you’ll learn

Safe SubstitutionCorrect InheritanceReliable Polymorphism

Page sections

01 What LSP Requires 02 Violation Signs 03 Rectangle / Square Problem 04 Custom Post Types 05 Widget Hierarchy 06 Common Violations 07 FAQ
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I Interface Segregation

SOLID — 04

Interface Segregation

Many specific interfaces over one general

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SOLID — 04 / Flow

I

Interface Segregation

No client should be forced to depend on methods it does not use.

Worker (fat interface)
BAD — work + eat + sleep — Robot forced to implement all
↓ Split by ISP
Workable
OK — work()
Feedable
OK — eat()
Restable
OK — sleep() + takeBreak()

What you’ll learn

Focused InterfacesNo Forced DependenciesBetter Composition

Page sections

01 What ISP Requires 02 Violation Signs 03 Worker / Robot Problem 04 Plugin Interfaces 05 Gutenberg Blocks 06 Common Violations 07 FAQ
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D Dependency Inversion

SOLID — 05

Dependency Inversion

Depend on abstractions, not concretions

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SOLID — 05 / Flow

D

Dependency Inversion

High-level modules should not depend on low-level modules.

UserService
depends on Database_Interface ← abstraction
Database_Interface
save() / find() / delete()
MySQL_Database
OK — implements interface
Mock_Database
OK — implements interface — for tests

What you’ll learn

Depend on AbstractionsSwappable ImplementationsEasy to Test

Page sections

01 What DIP Requires 02 Violation Signs 03 Database Dependency 04 Plugin: Mailer & Cache 05 DI Container 06 Common Violations 07 FAQ
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