如何在一个php页面中创建2个或更多PDO对象？

Hi I have a code which there is a pdo object like this :

$db = new PDO('mysql:host='.$servername.'; dbname='.$db.';charset=utf8' , $db_username, $db_password);

thats good but When I want to creat another PDO object wiht a different name i into same page like this :

$dbh = new PDO('mysql:host='.$servername.'; dbname='.$db.';charset=utf8' , $db_username, $db_password);
$notification_array = $dbh -> prepare("select * from `machines`ORDER BY id DESC limit 51 ");

php says :

 Catchable fatal error: Object of class PDO could not be converted to string

I thoght maybe I must make last PDO object ($db) NULL ! like this :

$db = null;
$dbh = new PDO('mysql:host='.$servername.'; dbname='.$db.';charset=utf8' , $db_username, $db_password);
$notification_array = $dbh -> prepare("select * from `machines`ORDER BY id DESC limit 51 ");

but by this way my code does not work and does not read table how can I create 2 simllar Pdo objects in a page ? I need it to keep them next to eachother

in $db = new PDO('mysql:host='.$servername.'; dbname='.$db.';charset=utf8' , $db_username, $db_password); you are using the same variable name $db for the pdo object and for the dbname You should use dbname='.$dbname for example.

Name your first connection $con or something, as you're using the $db variable for your database name already.

If you're connecting to the same database, and judging by using the same variables it looks like you are, you don't need to close your connection and reopen it in between queries.

$con = new PDO($db,$name,$pass);
$con->query();
//do what you want with your query

$con->query();
//and then just move on to your next query