just a php/mysql logic question

Hi All, Just sitting here thing about a php app i am making and an issue i have been aware of but never looked into has made itself apparent to me. Its to do with my php app and transacting with the mysql table of which my records are kept. note: sorry if this goes or is of topic. I have a table which holds a number of items in stock. If multiple users hit the php app at the same time how do i ensure that the correct amount of stock is taken from stock so that a users does not accidentally sell from stock which has already been sold. I have googled but to be honest i dont think i know the correct terminology to use for this kind of issue so hence the post here to the clever people of this great list. I thank you in advance for any assistance you may give. Dave C