---
id: php-functions
title: "PHP Functions"
category: "Programming_Language"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["variadic functions", "strict_types", "PHP 함수"]
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", "functions"]
raw_sources: ["https://www.w3schools.com/php/php_functions.asp"]
applied_in: []
github_commit: ""
---

# [[PHP Functions]]

## 🎯 한 줄 통찰 (One-line insight)
The `...` variadic operator can ONLY be the LAST parameter — putting it first (`function myFamily(...$firstname, $lastname)`) is explicitly shown as an ERROR case, because a variadic parameter greedily collects all remaining arguments into an array, leaving nothing for any parameter declared after it. [S1]

## 🧠 핵심 개념 (Core concepts)
- **1000+ built-in functions** available directly. [S1]
- **User-defined function** — `function name($params) { ...; return $value; }`; not case-sensitive; not auto-run (must be called). [S1]
- **Default parameter values** — `function setHeight($height = 50) {...}` — used when the argument is omitted. [S1]
- **Pass by reference** — `function add_five(&$value) { $value += 5; }` — the `&` makes changes persist back to the caller's variable (default is pass-by-value, a copy). [S1]
- **Variadic functions (`...`)** — `function sumMyNumbers(...$x) {...}` — accepts an unknown number of arguments, collected into an array; must be the LAST parameter. [S1]
- **Loosely typed by default** — no type declarations required. [S1]
- **`declare(strict_types=1);`** (PHP 7+) — must be the FIRST line of the file; enforces strict type checking, throwing a Fatal Error on type mismatch. [S1]
- **Return type declarations** — `function addNumbers(float $a, float $b) : float {...}` — colon syntax before the opening brace. [S1]

## 📖 세부 내용 (Details)
- Default parameter: `function setHeight($height = 50) { echo "The height is : $height <br>"; } setHeight(350); setHeight(); // uses default 50`. [S1]
- Pass by reference: `function add_five(&$value) { $value += 5; } $num = 2; add_five($num); echo $num; // 7`. [S1]
- Variadic (valid, last position): `function sumMyNumbers(...$x) { $n = 0; $len = count($x); for($i = 0; $i < $len; $i++) { $n += $x[$i]; } return $n; }`. [S1]
- Variadic (invalid, first position — errors): `function myFamily(...$firstname, $lastname) {...} // error: variadic must be last`. [S1]
- Strict types: `<?php declare(strict_types=1); function addNumbers(int $a, int $b) { return $a + $b; } echo addNumbers(5, "5 days"); // Fatal Error — "5 days" is not an int`. [S1]
- Return type declaration: `function addNumbers(float $a, float $b) : float { return $a + $b; }`. [S1]

## ⚖️ 모순 및 업데이트 (Contradictions & updates)
- **가변 인자의 위치 제약**: ... 연산자를 사용하는 매개변수는 반드시 마지막에 위치해야 하며, 그렇지 않으면 에러가 발생한다는 점이 실패 예제로 직접 검증됨. [S1]

## 🛠️ 적용 사례 (Applied in summary)
현재 발견된 실제 적용 사례가 없습니다 — 개수가 정해지지 않은 숫자들의 합을 구하는 가변 인자 함수(sumMyNumbers)가 실전 활용의 대표 사례다. [S1]

## 💻 코드 패턴 (Code patterns)
Variadic function summing an unknown number of arguments (PHP):
```php
function sumMyNumbers(...$x) {
    $n = 0;
    $len = count($x);
    for ($i = 0; $i < $len; $i++) {
        $n += $x[$i];
    }
    return $n;
}
$a = sumMyNumbers(5, 2, 6, 2, 7, 7);
echo $a;
```

## ✅ 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 Arrays]], [[PHP OOP Classes Objects]], [[PHP Callback Functions]]
- **참조 맥락:** 함수 정의/호출의 전체 문법 — 배열(Arrays) 챕터로 이어짐.

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

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