Yup I'm actually very comfortable with PHP but I've never heard of PDO before lol. It's really good to know that the PHP guys introduced an abstraction layer. I should check that out. Usually, when I think about scalability I try to abstract things as much as possible. Rather than creating a "Database" class, I would create a "MySQLDatabase" class and include all the logic which is tightly coupled with MySQL inside it. This way, I could easily create an "OracleDatabase" class and so on. But to be honest, I never had to work with anything other than MySQL when using PHP. I heard about PostgreSQL before but I never felt like I actually need it with MySQL already available to me.
EDIT:
@rahmu: In case you read the question I asked, do not answer it. I will start a new thread which should be interesting...
To everyone interested about PDO read this great
tutorial