try {
    $dbh= new PDO('mysql:host=serverName;dbname=Mydatabase','user','password');
    $dbh->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
}
catch (Exception $e) {
    die('Erreur : ' . $e->getMessage());
}

var_dump($dbh);    // gives : object(PDO)#1 (0) { }

$res=$dbh->query('SELECT * FROM table');

$res=$dbh->query('SELECT * FROM table');

$res=$dbh->query('SELECT * FROM Mydatabase.table');

$dbh= new PDO('mysql:host=serverName;dbname=Mydatabase','user','password');

$servername = "servername";
$username = "username";
$password = "password";
$dbname = "dbname";

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

    echo "Connection ok!";
    } catch (PDOException $e) {
    echo "Err: " . $e->getMessage();
    }

    $conn = null;