PHP 遍歷多維陣列兩大核心方法：遞迴函數與雙層 foreach 實戰比較

PHP 遍歷多維陣列實戰：雙層 foreach vs 遞迴函數完整比較

處理多維陣列是高頻需求。本文展示兩種核心寫法：釐清已知結構用雙層 foreach、結構不定深度用遞迴函數。

測試資料結構

<?php
$a = [
    'fruits'  => ['a' => 'orange', 'b' => 'grape', 'c' => 'apple'],
    'numbers' => [1, 2, 3, 4, 5, 6],
    'holes'   => ['first', 5 => 'second', 'third'],
];

方法一：雙層 foreach（已知固定二維結構）

<?php
foreach ($a as $listKey => $things) {
    if (is_array($things)) {
        foreach ($things as $newKey => $value) {
            echo "key: $newKey
value: $value
";
        }
    }
}

• 優點：直觀、效能最好、程式碼簡短
• 缺點：只能處理固定二維，無法應對更深層巢狀
• 適用：API 回傳固定結構、設定檔解析等

方法二：遞迴函數扁平化（不定深度、任意結構）

<?php
function flattenArray($array, &$result = []) {
    foreach ($array as $key => $value) {
        if (is_array($value)) {
            flattenArray($value, $result);
        } else {
            $result[] = $value;
        }
    }
    return $result;
}

$flat = flattenArray($a);
print_r($flat);
/*
Array
(
    [0] => orange
    [1] => grape
    [2] => apple
    [3] => 1
    [4] => 2
    [5] => 3
    [6] => 4
    [7] => 5
    [8] => 6
    [9] => first
    [10] => second
    [11] => third
)
*/

• 優點：處理任意深度巢狀陣列
• 缺點：遞迴開銷大、丟失鍵名資訊（可改為保留 key 路徑）
• 適用：深度未知的樹狀資料、表單欄位遞迴驗證

進階：保留鍵名路徑的遞迴遍歷

<?php
function walkArray($array, $path = '', &$result = []) {
    foreach ($array as $key => $value) {
        $currentPath = $path ? "$path.$key" : (string)$key;
        if (is_array($value)) {
            walkArray($value, $currentPath, $result);
        } else {
            $result[$currentPath] = $value;
        }
    }
    return $result;
}

$walked = walkArray($a);
/*
Array
(
    [fruits.a] => orange
    [fruits.b] => grape
    [fruits.c] => apple
    [numbers.0] => 1
    ...
)
*/

效能與選擇建議

PHP 內建替代方案

• array_walk_recursive()：內建遞迴遍歷，效能優於自寫遞迴
• RecursiveArrayIterator + RecursiveIteratorIterator：SPL 迭代器，物件導向風格

<?php
// 內建遞迴遍歷範例
$result = [];
array_walk_recursive($a, function($value, $key) use (&$result) {
    $result[] = $value;
});