Object-Oriented PHP · Lesson 1 of 14

What is OOP?

Model related state and behavior with objects and classes.

Lesson

Object-oriented programming groups state and behavior into objects created from classes. A class can enforce rules around its data and expose meaningful operations. Not every small script needs objects; introduce them when a domain concept has cohesive state, behavior, or collaborators. This first example gives a small value object one clear responsibility. 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 What is OOP?. 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

What is OOP?

PHP

class Counter
{
    public function next(int $value): int
    {
        return $value + 1;
    }
}
echo (new Counter())->next(4);

Expected output

Key points

Model related state and behavior with objects and classes.
Protect object invariants behind a small public API.
Prefer composition unless inheritance expresses a genuine subtype.

Your learning progress

Finish this lesson

Optional practice

PHP playground

This site does not execute learner-provided PHP code. Connect a secured PHP sandbox service before enabling code execution.

Execution is disabled. No code is sent to a server.