Decrease an integer field by 1 after successful login - what is the php?

I have a table called mcat_form, which holds data for each user of my project. One of the fields is "counter". Counter is initially set to 25 but I want it reduced by 1 every time the user successfully logs in.

What would the php code be to do this?

I assume that I am already logged in to the mySQL, since that is built into the project. I understand that the code should go in Events, under Log In Page/After successful log in.

Thanks anybody!