PDO 操作mysql 报错 QLSTATE[3D000]: 1046 No database selected

$dsn = "mysql:host = $host;dbname = $dbname;charset = UTF8"; try{ $dbh = new PDO($dsn,$user,$password);
echo'连接成功'.'<br/>';
} catch(PDOException $e) {
exit('数据库连接失败:'.$e->getMessage());
$dbh->setAttribute(PDO::ATTR_ERRMODE,PDO::ERRMODE_EXCEPTION);
$sql = "insert into test (name,sex,age) values('xi','femal','30')";$dbh->exec($sql);
}
PHP版本7.0 .1 mysql版本 5.7.11
用mysqli_connect 操作，可以增删改查操作数据库，证明数据库名等没有问题。用PDO 操作报错SQLSTATE[3D000]: Invalid catalog name: 1046 No database selected。
求大神解答