mysql_db_query and the future?

Hi folks, just wondering, why the mysql_db_query is being abandoned and called "deprecated" ? With the current implementation of the mysql_connect I have a problem with using 2 databases with the same username/password. PHP will use the same connection and so only use one of the two databases :-(. Is there any other way than always calling mysql_select_db before making a query? I was used using an abstraction layer with a class for each connection which used also mysql_db_query for the query so that I could use each class on a different database (but with the same connection parameters). Is there a speed penalty in using the combination of mysql_select_db();mysql_query ... compared against mysql_db_query? Thanks a lot for already reading this ;-) Thomas