Object-Oriented PHP · Lesson 8 of 14

Inheritance

Extend a base class only when the subtype can honor its contract.

Lesson

Inheritance lets a child class reuse or specialize behavior from a parent. Use it for a genuine is-a relationship, not merely to share a few lines. Prefer composition when behavior can be delegated, because deep inheritance chains make changes harder to reason about. The child inherits a stable method and adds one focused capability. Further guidance Give an object one clear responsibility and protect state that must satisfy a rule. Prefer a small public contract and composition for replaceable collaborators; use inheritance only for a genuine subtype. Test behavior through public methods instead of relying on internal implementation details. Practice task: Create a fresh example for Inheritance. Predict the result before running it, then change one input and explain why the result changes. Test a normal case and one boundary or invalid case where it applies. Compare your output with the example and note one rule you would use in a real project.

Code examples

Inheritance

PHP

class Message
{
    public function text(): string { return 'Hello'; }
}
class WelcomeMessage extends Message {}
echo (new WelcomeMessage())->text();

Expected output

Hello

Key points

Extend a base class only when the subtype can honor its contract.
Protect object invariants behind a small public API.
Prefer composition unless inheritance expresses a genuine subtype.

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