How to lock a transaction in php

I am designing a web application in php which involves 1 server and multiple clients.

I am using Xaamp Server as this web application is to be executed on LAN only.

Now, the problem is - These clients will connect to the server and will try to update a value using the IP address of that server. There will be login for clients so session will be managed accordingly. What i want is - Out of N clients only 1 client must be able to update a value (For example, if server issues a share of $1000, now the first client to buy that share must get that and the other clients will get an acknowledgement that "Sorry got sold out") Hope you got my problem. Please help me as soon as possible.