Assume you have 2 database in one mysql server:

Database1 (contains table: table1)
Database2 (contains table: table2)

Try this:
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$conn_id1 = mysql_connect("localhost", "root", "");
$conn_id2 = mysql_connect("localhost", "root", "");

mysql_select_db("Database1", $conn_id1);
mysql_select_db("Database2", $conn_id2);

mysql_query("select * from table1", $conn_id1); 
mysql_query("select * from table2", $conn_id2);

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the first query will fail, b/c the 'select_db' for conn_id1 was overwritten in the 'select_db' for conn_id2: it is looking for table1 in Database2.

This hold true even inside objects, so it is difficult to create an object that accesses the database and use it in a script that accesses a different database.