[Dovecot] Store mailbox usage in mysql not just the limit

[tibby]

Hello!

I have dovecot working using mysql for auth and mailbox quota.
in the mail database I have a user table with colums for:
email, password, quota_kb, quota_messages

I want to make a php script that would get the quota_kb and substract 
the usage to display the total free space left for mail usage.
How can I do this?
Is there a function in dovecot to store the current usage per mailbox ?

Thanks!