测试数组

$array = [
    ['id' => 1, 'pid' => 0, 'name' => '这是主类'],
    ['id' => 2, 'pid' => 0, 'name' => '这是主类'],
    ['id' => 3, 'pid' => 1, 'name' => '父级为1子类'],
    ['id' => 4, 'pid' => 2, 'name' => '父级为2子类'],
    ['id' => 5, 'pid' => 3, 'name' => '父级为3子类'],
];

一、递归算法

function tree($array, $pid)
{
    $tree = array();
    foreach ($array as $key => $value) {
        if ($value['pid'] == $pid) {
            $value['child'] = tree($array, $value['id']);
            if (!$value['child']) {
                unset($value['child']);
            }
            $tree[] = $value;
        }
    }
    return $tree;
}

echo "<pre>" . print_r(tree($array, 1), 1) . "</pre>";

二、数组引用

function generateTree($rows, $id = 'id', $pid = 'pid')
{
    $items = array();
    foreach ($rows as $row) {
        $items[$row[$id]] = $row;
    }
    foreach ($items as $item) {
        $items[$item[$pid]]['son'][$item[$id]] = &$items[$item[$id]];
    }
    return isset($items[0]['son']) ? $items[0]['son'] : array();
}

echo "<pre>" . print_r(generateTree($array), 1) . "</pre>";

看不懂的萌新可以使用PhpStorm在输出那儿打断点进行调试哈。