how to make a query with database as a variable

I have two databases – lorem and nts.lorem – and need to operate with both of them

$user = 'root';
$pass = '';
$db1 = new PDO('mysql:host=localhost; dbname=nts.lorem', $user, $pass);
$db2 = new PDO('mysql:host=localhost; dbname=lorem', $user, $pass);

everything works fine until db is a variable in an ajax request – for example:

js

var db;
if(something is true){db = 'db1';};
else{db = 'db2';}
//... ajax post code

php

function something($db){
    global $db1, $db2;
    // how to say the next line  
    $sq = "select id from " . $db . ".tableName order by title asc";
    // error - table db1.tableName doesn't exist  
}

any help?

Answer

Choose connection according to $db value:

function something($db){
    global $db1, $db2;
    $sq = "select id from tableName order by title asc";
    if ($db === 'db1') {
        $db1->execute($sq);
    } else {
        $db2->execute($sq);
    }    
    // rest of the code  
}