php queston

I'm saving some text areas to a database and if they are empty I'm ending up with '0' in the field in the DB.

the default value is null for the field (mysql).  How can I format the variable before sending it to the database to not show zero.. can I insert a null value?  

My alternative is to live with the zeros and handle them either in the view php (mvc) or in a jquery script to replace with and empty string.  Seems kind of cheesy to do it that way though.

How can I do this properly in php before I send it to the db?

(FYI it's php within CodeIgniter)