如何使用PHP使用多个数据库？

$mysqli = new mysqli("localhost", "username", "password", "database1");

// 检查连接是否成功
if ($mysqli->connect_errno) {
    echo "连接数据库失败：" . $mysqli->connect_error;
    exit();
}

$mysqli2 = new mysqli("localhost", "username", "password", "database2");

// 检查连接是否成功
if ($mysqli2->connect_errno) {
    echo "连接数据库失败：" . $mysqli2->connect_error;
    exit();
}

$query1 = "SELECT * FROM table1";
$result1 = $mysqli->query($query1);

// 处理查询结果
while ($row = $result1->fetch_assoc()) {
    // 处理数据
}

$query2 = "SELECT * FROM table2";
$result2 = $mysqli2->query($query2);

// 处理查询结果
while ($row = $result2->fetch_assoc()) {
    // 处理数据
}

$mysqli->close();
$mysqli2->close();