PHP SimpleXML属性访问：理解与正确处理其返回类型

<gesmes:Envelope xmlns:gesmes="http://www.gesmes.org/xml/2002-08-01" xmlns="http://www.ecb.int/vocabulary/2002-08-01/eurofxref">
    <gesmes:subject>Reference rates</gesmes:subject>
    <gesmes:Sender>
        <gesmes:name>European Central Bank</gesmes:name>
    </gesmes:Sender>
    <Cube>
        <Cube time="2021-12-13">
            <Cube currency="USD" rate="1.1278"/>
            <Cube currency="JPY" rate="128.19"/>
            <!-- ... 更多货币 ... -->
        </Cube>
    </Cube>
</gesmes:Envelope>

<?php
$xmlString = <<<XML
<gesmes:Envelope xmlns:gesmes="http://www.gesmes.org/xml/2002-08-01" xmlns="http://www.ecb.int/vocabulary/2002-08-01/eurofxref">
    <gesmes:subject>Reference rates</gesmes:subject>
    <gesmes:Sender>
        <gesmes:name>European Central Bank</gesmes:name>
    </gesmes:Sender>
    <Cube>
        <Cube time="2021-12-13">
            <Cube currency="USD" rate="1.1278"/>
            <Cube currency="JPY" rate="128.19"/>
            <Cube currency="BGN" rate="1.9558"/>
        </Cube>
    </Cube>
</gesmes:Envelope>
XML;

$xml = new \SimpleXMLElement($xmlString);

// 遍历最内层的Cube元素
foreach ($xml->Cube->Cube->Cube as $c) {
    // 访问属性
    $currency = $c->attributes()->currency;
    $rate = $c->attributes()->rate;

    echo "Currency: " . $currency . ", Rate: " . $rate . PHP_EOL;
}
?>

<?php
// ... (接上文的 $xml 和 foreach 循环) ...

foreach ($xml->Cube->Cube->Cube as $c) {
    $currencyAttributeObject = $c->attributes()->currency;

    // 打印属性对象的名称
    echo "Attribute Name: " . $currencyAttributeObject->getName() . PHP_EOL; // 输出 'currency'

    // 尝试直接打印对象，PHP会进行隐式转换
    echo "Attribute Value (implicit): " . $currencyAttributeObject . PHP_EOL; // 输出 'USD'
}
?>

public function setCurrencyRate($currency, $rate)
{
    // 在这里，$currency 和 $rate 仍然是 SimpleXMLElement 对象
    // 如果没有显式转换，当它们被用于需要字符串的上下文时，可能会变成空字符串
    // 例如，某些数据库驱动或ORM在处理 SimpleXMLElement 对象时，
    // 可能无法正确提取其值，或者将其视为非字符串类型导致错误。
    $data = [
        'currency' => $currency,  // 此时可能是空字符串或类型错误
        'rate' => $rate,          // 此时可能是空字符串或类型错误
        'updated' => new \DateTime(),
    ];
    // 假设 db->query 期望字符串参数
    // $this->db->query('INSERT INTO currencies_rates ...', $data);
}

// 调用时：
// $this->currency->setCurrencyRate($c->attributes()->currency, $c->attributes()->rate); // 错误：传入的是对象，而非字符串

<?php
// 假设 $this->currency 是一个服务对象
// ... (接上文的 $xml 和 foreach 循环) ...

foreach ($xml->Cube->Cube->Cube as $c) {
    // 使用 (string) 进行显式类型转换
    $currencyString = (string) $c->attributes()->currency;
    $rateString = (string) $c->attributes()->rate;

    // 现在传入函数的是真正的字符串
    $this->currency->setCurrencyRate($currencyString, $rateString);
}

// 假设 setCurrencyRate 函数定义如下
class CurrencyService {
    private $db; // 假设有一个数据库连接

    public function __construct($dbConnection) {
        $this->db = $dbConnection;
    }

    public function setCurrencyRate($currency, $rate)
    {
        // 此时 $currency 和 $rate 确定是字符串类型
        $data = [
            'currency' => $currency,
            'rate' => $rate,
            'updated' => (new \DateTime())->format('Y-m-d H:i:s'), // 格式化日期以适应数据库
        ];

        // 示例：模拟数据库插入或更新操作
        echo "Inserting/Updating: Currency = {$data['currency']}, Rate = {$data['rate']}, Updated = {$data['updated']}" . PHP_EOL;

        // 实际的数据库操作可能如下：
        // $this->db->query(
        //     'INSERT INTO currencies_rates (currency, rate, updated) VALUES (?, ?, ?) 
        //      ON DUPLICATE KEY UPDATE rate = VALUES(rate), updated = VALUES(updated)',
        //     [$data['currency'], $data['rate'], $data['updated']]
        // );
    }
}

// 模拟数据库连接和 CurrencyService 实例化
$mockDb = new class { public function query(...$args) {} };
$this->currency = new CurrencyService($mockDb);

// 重新运行解析逻辑
// ... (接上文的 $xml 和 foreach 循环) ...
foreach ($xml->Cube->Cube->Cube as $c) {
    $this->currency->setCurrencyRate(
        (string) $c->attributes()->currency, 
        (string) $c->attributes()->rate
    );
}
?>