Creating a scalable service on MySQL

I am in the process of creating a scalable service in cakePHP and at this current point of time I have a working prototype using one database.

My question is what is the best way to move forward in regards to signing up clients and keeping their data separate.

Each client has their own database? This was my only thought as I am not interested in having data within the same table and prefixing will simply become messy.

Thoughts?

Thanks all