There is no such option provided in RDS API. When you call CreateDBInstance API, you can specify the DBName
but that is at the time of creating a new RDS instance. Once you have created the instance, then there is no provision to create a new DB on it.
I would imagine you need to connect via PHP using ODBC (or whatever that PHP supports) to the RDS endpoint and then create a new DB just like you are connecting to a non-RDS remote DB.