Hi.
If a pool service says that it (for example) "hashes 200Gh/s", does that mean that it receives 200 billions of requests with users' hashes per second?
I ask because I would like to know if this is sensible to create a pull service on PHP.
No, it means that its miners do that many hashes.
If the difficulty of a share is 1, then each share submitted represents 4 billion hashes completed by the miner (on average).
They would out their hash rate as 2
32 * (share difficulty) * (shares submitted per second).
The pool can decide how high it sets the share difficulty. The lower the less variance for the miners, but the more server resources it uses.
It can be different per miner. If a miner submits shares every second, the pool could increase the share difficulty for that miner. By doing this, each miner's share rate could be adjusted so it is once every few minutes.