mysql_pconnect - persistance scope?

I have a question about mysql_pconnect. I execute it in one file logon.php and successfully do some db queries, later on in another file xyz.php I do some other queries but they fail. It works OK if I repeat the mysql_pconnect() and the mysql_select_db() in that file, is the persistance only across a single script? i.e. do I have to do a pconnect for every php file that does db access? If I do, how do I store the original userid and password to make them global to all php scripts. If I don't then what could the problem be? Some kind of timeout? Thanks John.