Hi!
It is possible in PHP to use mySQL database from another hosting account. Let me explain...
My script in abc.com and I want to use database from xyz.com.
is this possible? and how

Yes, as long you obtain permission connecting to the database remotely.

It is possible.

Lets say you want to connect from server A to server B.

Server A should use server's B ip address or hostname in order to connect.

Administrator of the server B should give permission for server A to access the database(s).

Adding New Users to MySQL
http://www.mysql.com/doc/en/Adding_users.html