PHP - Connect to Multiple MySQL Databases

You are not mentioning database connection link while executing the query. May be your second database connection is defined after the 1st one in connection.php file. So interpreter is considering 2nd connection while executing the query.

Define the connection link with query,

$thing = mysql_query($thing,$conn); //$conn is your mysql_connect

You can still use the mysql_connect or similar mysql_ functions (procedural way) But all these are now deprecated. You should use mysqli_ (mysql improved) functions or go with the object oriented way.

$conn1 = new mysqli(SERVER1,DB_USER1,DB_PASS1,DB1);
$conn2 = new mysqli(SERVER2,DB_USER2,DB_PASS2,DB2);

if($conn1 ->connect_errno > 0 || $conn2 ->connect_errno > 0){
    die('Unable to connect to database server.');
}

Now while executing your query,

$query = "SELECT abc, efg, hij FROM table ORDER BY RAND() LIMIT 1";
$thing = $conn1 -> query($query); //if it's second connection query user $conn2

Many web applications benefit from making persistent connections to database servers. Persistent connections are not closed at the end of the script, but are cached and re-used when another script requests a connection using the same credentials. The persistent connection cache allows you to avoid the overhead of establishing a new connection every time a script needs to talk to a database, resulting in a faster web application.

For your case, i would make 2 persistent connections, store each in it's own variable and then use one whenever i need to. Check it out using the PDO class.

//Connection 1
$dbh1 = new PDO('mysql:host=localhost;dbname=test', $user, $pass, array(
    PDO::ATTR_PERSISTENT => true
));

.
.
.

//Connection 2
$dbh2 = new PDO('mysql:host=localhost;dbname=test2', $user, $pass, array(
    PDO::ATTR_PERSISTENT => true
));

After you finish you first database operation, then you can close the connection using

mysql_close($c) //$c = connection

And again start for next database operation.

private static function _connectDB1()
{
//give hostname, dbusername, dbpassword
$con1 = mysql_connect("localhost", "root", "rootpass");
$db1 = mysql_select_db("dbone", $con1);
}
private static function _connectDB2()
{
//if you are using different db with different credentials
//you can give here like this
// mysql_connect("mynewhost", "mynewusername", "mynewpassword")
$con2 = mysql_connect("localhost", "root", "rootpass");
$db2 = mysql_select_db("dbtwo", $con2);
}