As PHP is mentioned in the question,I think @duckx want to make similar service like ZMQ or -walletnotify so I did not stated that earlier.

extremetech.com/extreme/… This article may help you to understand it.

congrats!!, in that case you can use cronjobs easily to monitor them.

There are Trillions of possible public key, so it is not viable solution to monitor each and every key. But it is usable if we have a defined limited set of public key from some user. we can use this cronjobs to allow to track Five or Six public key per user for thousand of users without any overhead on Server.

To forge double transaction, a user must have at least 51% computing power of the Bitcoin network(That is millions of miner and mining pools combined) to outperform the others in calculations and make his block chain longer. That is impossible!!! so we can Surely say the Early transaction(with longer blockchain) is the valid one.

That has to be done using cron jobs to monitor the blockchain scanning third party server(blockchain.info) or local wallet installation.