即使已连接，PHP也无法选择数据库

<?php
$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "database";

// 创建连接
$conn = mysqli_connect($servername, $username, $password);

// 检查连接是否成功
if (!$conn) {
    die("连接失败: " . mysqli_connect_error());
}

// 选择数据库
if (!mysqli_select_db($conn, $dbname)) {
    die("选择数据库失败: " . mysqli_error($conn));
}

echo "成功选择数据库";
?>

<?php
$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "database";

try {
    $conn = new PDO("mysql:host=$servername;dbname=$dbname", $username, $password);
    $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    echo "连接成功";

    // 选择数据库
    $conn->exec("USE $dbname");
    echo "成功选择数据库";
} catch(PDOException $e) {
    echo "连接失败: " . $e->getMessage();
}
?>