添加到mysql中的现有值

I am a newbie to mysql. I am making a table named inventory with 4 columns- id,name, price,quantity. Now if name already exists, I am to add the quantity to existing one else create a new row. (name is defined as UNIQUE). I am doing this on php. $name, $quant,$price contain the values.I am a newbie to mysql. I am making a table nam