PHP 多维数组：嵌套数组与遍历

多维数组是元素本身也是数组的数组。每一层用一对方括号读取：$grid[1][2] 是第 1 行第 2 列，$users[0]['name'] 是第一条记录中的名字。

<?php
$grid = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9],
];

echo $grid[1][2], "\n";   // row 1, column 2

$users = [
    ['name' => 'Ada',   'role' => 'admin'],
    ['name' => 'Linus', 'role' => 'editor'],
];

echo $users[0]['name'], "\n";
echo $users[1]['role'], "\n";

$grid[2][0] = 70;         // change a nested value
echo implode(' ', $grid[2]), "\n";

第二种形式，即由带命名字段的记录组成的列表，是你最常遇到的：数据库查询、解码后的 JSON 响应或解析后的 CSV 文件通常给你的就是它。每条记录都是一个 关联数组。

创建嵌套数组

嵌套可以按需要任意加深，各层也可以混用数字键和字符串键。配置数组是一个常见的例子：一些有名字的部分，每部分包含若干设置或列表。

<?php
$config = [
    'db' => [
        'host' => 'localhost',
        'port' => 3306,
    ],
    'admins' => ['ada', 'grace'],
    'features' => [
        'search' => ['enabled' => true, 'limit' => 20],
    ],
];

echo $config['db']['host'], "\n";
echo $config['admins'][1], "\n";
echo $config['features']['search']['limit'], "\n";

添加行和嵌套的值

$rows[] = [...] 追加一整行。$arr['key'][] = $value 在嵌套的列表中追加，第一次时 PHP 会自动创建内层数组，所以给值分组不需要任何准备工作。

<?php
$orders = [
    ['customer' => 'Ada',   'item' => 'book'],
    ['customer' => 'Linus', 'item' => 'pen'],
    ['customer' => 'Ada',   'item' => 'lamp'],
];

$orders[] = ['customer' => 'Grace', 'item' => 'mug'];   // add a row

$byCustomer = [];
foreach ($orders as $order) {
    $byCustomer[$order['customer']][] = $order['item'];  // inner array created on first use
}

print_r($byCustomer);

遍历多维数组

每一层用一个 foreach。对于由记录组成的列表，一个循环通常就够了，因为你会按名称读取字段。对于网格，内层循环会依次访问每一行中的单元格。

<?php
$students = [
    ['name' => 'Ada',   'scores' => [90, 85, 99]],
    ['name' => 'Linus', 'scores' => [70, 88, 79]],
];

foreach ($students as $student) {
    $average = array_sum($student['scores']) / count($student['scores']);
    echo $student['name'], ": ", round($average, 1), "\n";
}

$grid = [[1, 2, 3], [4, 5, 6]];
foreach ($grid as $r => $row) {
    foreach ($row as $c => $cell) {
        echo "[$r][$c]=$cell ";
    }
    echo "\n";
}

用数组生成 HTML 表格

由行组成的数组可以直接对应到 HTML 表格：第一行的键成为表头，每一行成为一个 <tr>。运行后，“页面”标签页会显示渲染出来的表格；修改一个值或添加一行，再运行一次。

<?php
$products = [
    ['name' => 'Keyboard', 'price' => 49.99, 'stock' => 12],
    ['name' => 'Mouse',    'price' => 19.50, 'stock' => 0],
    ['name' => 'Monitor',  'price' => 189.00, 'stock' => 4],
];
?>
<style>
  table { border-collapse: collapse; font-family: sans-serif; }
  th, td { border: 1px solid #ccc; padding: 6px 12px; text-align: left; }
  .out { color: #b00; }
</style>
<table>
  <tr>
    <?php foreach (array_keys($products[0]) as $heading): ?>
      <th><?= htmlspecialchars(ucfirst($heading)) ?></th>
    <?php endforeach; ?>
  </tr>
  <?php foreach ($products as $product): ?>
    <tr class="<?= $product['stock'] === 0 ? 'out' : '' ?>">
      <td><?= htmlspecialchars($product['name']) ?></td>
      <td><?= number_format($product['price'], 2) ?></td>
      <td><?= $product['stock'] ?></td>
    </tr>
  <?php endforeach; ?>
</table>

foreach (...): ... endforeach; 这种写法是专为模板准备的同一种循环，能让 HTML 保持可读。

从所有行中取出一列

array_column() 把每一行中的某个字段取出来，组成一个扁平的列表。传入第三个参数可以用另一个字段作为结果的键，这样就把记录列表变成了一张查找表。

<?php
$users = [
    ['id' => 3, 'name' => 'Ada',   'email' => 'ada@example.com'],
    ['id' => 7, 'name' => 'Linus', 'email' => 'linus@example.com'],
    ['id' => 9, 'name' => 'Grace', 'email' => 'grace@example.com'],
];

print_r(array_column($users, 'name'));
print_r(array_column($users, 'email', 'id'));   // id => email

$byId = array_column($users, null, 'id');       // id => whole row
echo $byId[7]['name'], "\n";

按字段对行排序

usort() 用一个比较函数对行进行排序。太空船运算符 <=> 返回 -1、0 或 1，这正是 usort() 所期望的。交换 $a 和 $b 即可按降序排序。

<?php
$players = [
    ['name' => 'Ada',   'points' => 120],
    ['name' => 'Linus', 'points' => 95],
    ['name' => 'Grace', 'points' => 140],
];

usort($players, fn($a, $b) => $b['points'] <=> $a['points']);   // highest first

foreach ($players as $rank => $player) {
    echo $rank + 1, ". {$player['name']} ({$player['points']})\n";
}

echo count($players), " rows, ", count($players, COUNT_RECURSIVE), " elements in total\n";

count($players, COUNT_RECURSIVE) 会统计 3 行再加上其中的 6 个字段，所以打印的是 9。

常见错误：读取可能不存在的深层键

在没有 cache 键的情况下读取 $config['cache']['ttl']，会先打印 Warning: Undefined array key "cache"，接着下一层还会打印 Warning: Trying to access array offset on null。?? 运算符会一次性检查整条链，而且从不警告，所以它是读取可选嵌套值的安全方法。

<?php
$config = ['db' => ['host' => 'localhost']];

$ttl  = $config['cache']['ttl'] ?? 60;          // missing at the first level
$port = $config['db']['port'] ?? 3306;          // missing at the second level
$host = $config['db']['host'] ?? '127.0.0.1';   // present

echo "$ttl $port $host\n";
var_dump(isset($config['db']['host']));