I\'m new to PDO. I would like to know if there is anything similar to mysql_select_db in PDO, so that i can switch between different databases during runtime without the nee
It looks like PDO does not have database switching because not every database engine supports it.
AFAIK PostgreSQL does not have database switching, but offer schemas and u can switch between those.
However if you're using mysql check if this works for you:
$pdo = new PDO('mysql:dbname=db1;host=127.0.0.1','user','pass');
$sql = 'select count(*) from table_name';
$res = $pdo->query($sql);
print_r($res->fetchAll());
$pdo->exec('USE db2');
$res = $pdo->query($sql);
print_r($res->fetchAll());