---
id: php-oop-inheritance
title: "PHP OOP Inheritance"
category: "Programming_Language"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["extends", "final keyword", "PHP 상속"]
duplicate_of: ""
source_trust_level: "B"
confidence_score: 0.9
created_at: 2026-07-04
updated_at: 2026-07-04
review_reason: ""
merge_history: []
tags: ["php", "programming", "w3schools", "oop", "inheritance"]
raw_sources: ["https://www.w3schools.com/php/php_oop_inheritance.asp"]
applied_in: []
github_commit: ""
---

# [[PHP OOP Inheritance]]

## 🎯 한 줄 통찰 (One-line insight)
Calling a `protected` method from OUTSIDE the class fails, but calling that SAME protected method from WITHIN a subclass's own method (via `$this->intro()`) succeeds — the exact rule from access modifiers applies identically to methods, and the source proves it with a direct pass/fail comparison of the two call sites. [S1]

## 🧠 핵심 개념 (Core concepts)
- **`extends`** — inherits public and protected properties/methods from a parent class. [S1]
- **Private members are NOT inherited** — a child class cannot access a parent's private methods/properties at all. [S1]
- **Method overriding** — redefining a method with the same name in the child class replaces the parent's version. [S1]
- **`final` on a class** — prevents any class from extending it. [S1]
- **`final` on a method** — prevents child classes from overriding that specific method. [S1]

## 📖 세부 내용 (Details)
- Basic inheritance: `class Fruit { public $name; public $color; public function __construct($name, $color) {...} public function intro() {...} } class Strawberry extends Fruit { public function message() {...} } $strawberry = new Strawberry("Strawberry", "red"); $strawberry->intro(); $strawberry->message();` — Strawberry uses Fruit's constructor/intro plus its own message(). [S1]
- Protected method fails from outside: `$strawberry->intro(); // ERROR if intro() is protected`. [S1]
- Protected method works from inside a child method: `public function message() { $this->intro(); }` — calling `$this->intro()` from within `message()` succeeds. [S1]
- Overriding: child class `Strawberry` redefines `__construct()` and `intro()` with the same names, completely replacing the parent's versions (adding a `$weight` property along the way). [S1]
- `final` prevents inheritance: `final class Fruit {...} class Strawberry extends Fruit {...} // error`. [S1]
- `final` prevents method override: `class Fruit { final public function intro() {...} } class Strawberry extends Fruit { public function intro() {...} // error }`. [S1]

## ⚖️ 모순 및 업데이트 (Contradictions & updates)
- **protected 메서드 호출 위치에 따른 성패**: 클래스 외부에서 호출하면 에러, 파생 클래스 내부 메서드에서 $this로 호출하면 성공한다는 점이 두 대비 예제로 직접 증명됨. [S1]

## 🛠️ 적용 사례 (Applied in summary)
현재 발견된 실제 적용 사례가 없습니다 — Strawberry가 Fruit의 생성자/메서드를 재정의(override)하며 weight 속성을 추가하는 것이 실전 오버라이딩의 대표 사례다. [S1]

## 💻 코드 패턴 (Code patterns)
Overriding a parent's constructor and method (PHP):
```php
class Fruit {
    public $name;
    public $color;
    public function __construct($name, $color) { $this->name = $name; $this->color = $color; }
    public function intro() { echo "The fruit is $this->name and the color is $this->color."; }
}
class Strawberry extends Fruit {
    public $weight;
    public function __construct($name, $color, $weight) {
        $this->name = $name; $this->color = $color; $this->weight = $weight;
    }
    public function intro() {
        echo "A $this->name is $this->color, and the weight is $this->weight gram.";
    }
}
```

## ✅ 검증 상태 및 신뢰도
- **상태:** draft
- **검증 단계:** conceptual
- **출처 신뢰도:** B (W3Schools — widely used educational reference, not a primary standards body)
- **신뢰 점수:** 0.90
- **중복 검사 결과:** 신규 생성 (New discovery)

## 🔗 지식 그래프 (Knowledge Graph)
- **상위/루트:** [[PHP Tutorial]]
- **관련 개념:** [[PHP OOP Access Modifiers]], [[PHP OOP Constants]], [[PHP OOP Classes Abstract]]
- **참조 맥락:** 클래스 상속과 메서드 오버라이딩 — 클래스 상수(Constants) 챕터로 이어짐.

## 📚 출처 (Sources)
- [S1] W3Schools — PHP OOP - Inheritance — https://www.w3schools.com/php/php_oop_inheritance.asp

## 📝 변경 이력 (Change history)
- 2026-07-04: Initial draft synthesized from the W3Schools "PHP OOP Inheritance" page (Astra wiki-curation, P-Reinforce v3.1 format).