starmyc is incorrect.
This is harsh (and incorrect too

).
You have BOTH:
and
blocknotify will execute whenever Bitcoin Core receives a new block.
walletnotify will execute whenever Bitcoin Core receives a transaction that is associated with an address that belongs to the wallet that Bitcoin Core is running with.
Between the two of those, along with the other RPC commands available, you should be able to build something robust.
The question is about calling back an URL, and those calls can only be used to launch an external program, not call an url as asked. You'll still need a 3rd party program to answer the initial need, as I proposed initially.