Connection failed: SQLSTATE[HY000] [2002] php_network_getaddresses: getaddrinfo failed: No such host is known. Fatal error: Uncaught Error: [closed]

Question

I'm a beginner in PHP and was trying to access my database after clicking on the login I get the error below while establishing connection

Connection failed: SQLSTATE[HY000] [2002] php_network_getaddresses: getaddrinfo failed: No such host is known. Fatal error: Uncaught Error: Call to a member function prepare() on string in C:\xampp\htdocs\simplesite\validate.php:17 Stack trace: #0 {main} thrown in C:\xampp\htdocs\simplesite\validate.php on line 17

Below is my validate.php code

<?php

include_once('connection.php');

function test_input($data) {
    
    $data = trim($data);
    $data = stripslashes($data);
    $data = htmlspecialchars($data);
    return $data;
}

if ($_SERVER["REQUEST_METHOD"]== "POST") {
    
    $adminname = test_input($_POST["adminname"]);
    $password = test_input($_POST["password"]);
    $stmt = $conn->prepare("SELECT * FROM admin_login");
    $stmt->execute();
    $users = $stmt->fetchAll();
    
    foreach($users as $user) {
        
        if(($user['adminname'] == $adminname) &&
            ($user['password'] == $password)) {
                header("Location: adminpage.php");
        }
        else {
            echo "<script language='javascript'>";
            echo "alert('WRONG INFORMATION')";
            echo "</script>";
            die();
        }
    }
}

?>

connection.php code

<?php

$conn = "";

try {
    $servername = "simplesite";
    $dbname = "simplesite";
    $username = "root";
    $password = "";

    $conn = new PDO(
        "mysql:host=$servername; dbname=simplesite",
        $username, $password
    );
    
$conn->setAttribute(PDO::ATTR_ERRMODE,
                    PDO::ERRMODE_EXCEPTION);
}
catch(PDOException $e) {
    echo "Connection failed: " . $e->getMessage();
}

?>