php: Creating automatic database sharding logic?

I did just come up with the following idea, but I lack the knowledge to say if it is applicable to a production application.

We have a web application, built on PHP/mySQL to make it easy. A table in the database are prone to grow large - a couple million of records easily, so table sharding might be an option here.

Here's how I have imagined the process to work:

A cached file contains the a list with the available tables in the database. Each table contains a maximum of a million rows and when that is reached, the cached list is recreated after a new table has been constructed.

Obviously it wouldn't be a good idea to check the number of rows on every write to the table, so this could be done on a set interval, like a week, or daily - depending on how quick every million of data is created.

Would this be a good way to deal with large amount of data and to keep index sizes fairly low?

Thanks