mysqlnumrows 没有返回值

$mysqli = new mysqli("localhost", "username", "password", "database");

if ($mysqli->connect_error) {
    die("连接失败: " . $mysqli->connect_error);
}

$sql = "SELECT * FROM table_name";
$result = $mysqli->query($sql);

if ($result) {
    $num_rows = $result->num_rows;
    echo "行数: " . $num_rows;
} else {
    echo "查询错误: " . $mysqli->error;
}

$mysqli->close();

try {
    $pdo = new PDO("mysql:host=localhost;dbname=database", "username", "password");
    $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

    $sql = "SELECT * FROM table_name";
    $result = $pdo->query($sql);

    $num_rows = $result->rowCount();
    echo "行数: " . $num_rows;
} catch (PDOException $e) {
    echo "连接失败: " . $e->getMessage();
}

$pdo = null;