add a column to a table via PHP

Hi, I'm probably being a bit lazy asking what I should be able to figure out, but here goes anyway: What I want to do, in response to certain other actions having taken place, is add a column to a mySQL database (FocusShop) table (Prices) and set the default value of that column and update all existing rows in that table to the same value for that new column. Has anyone got the correct way to do this? Thanks, Martin.