我有一个由联合域名托管的网站。它和mysqli配合得很好，但我把它改成了pdo。现在我无法让数据库工作。我尝试将$host改为127.0.0.1和localhost。如果我使用localhost，我会得到错误 SQLSTATE[HY000] [2002] No such file or directory .
如果我使用127.0.0.1 SQLSTATE[HY000] [2002] Connection refused .
我也尝试过更改排序规则，但仍然不起作用。它在我的另一个域hostinger上运行得非常好。

$host = "localhost"; // I have tried using both 127.0.0.1 and localhost here
$db = "db_name"; //db name goes here
$username = "username"; //username goes here
$password = "password"; //password goes 

try {
    //Creating a PDO instance
    $conn = new PDO("mysql:host=$host;dbname=$db", $username, $password);
    $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    $conn->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
    $conn->setAttribute(PDO::ATTR_DEFAULT_FETCH_MODE, PDO::FETCH_OBJ);

}catch(\PDOException $e){
    // if connection fails, show PDO error
    echo "Connection Failed: ". $e->getMessage(); 
}