First your node does not detect specific transaction, it just sync the blockchain and based on the configuration of your node, it computes transaction and update blockchain state, or simply sync.
To achieve what you want, you need to run a program watching interesting "events" related to your contract.
If your process need to be fault tolerant, you need to store blocks already processed, to be able to restart your program and get potential old events.
Based on this, is PHP really important ? you decide based on your knowledge of nodejs / php and the purpose of the project.
PHP is not really important : You just want simple a task, or call to an URI with PHP right after. You can just adapt a simple example in nodejs with web3js, and do you want when an event occur.
script + web3js >
simple task, or call to an external URL where your php script is executed
PHP is really important : you can use some implementation like this one https://github.com/digitaldonkey/ethereum-php and code the wanted behavior.
php script (with a php implementation) >
request all events since last launch >
do processes >
store last watched block in db >
wait for next cron
In both scenarios, you need to connect to an ethereum node. It's where your node comes to play.
But, you can also use public node, like infuria.io
Your node is just a "gateway" to the ethereum blockchain, nothing more, nothing less.