On some transactions the txid and hash have the same value as each other and on other transactions they have different values. Why is this?

I am writing a blockchain explorer in php to run off my local Bitcoin Core node. I am getting the RPC to work. I will need some calculated data like estimated btc transferred in a block or btc balance for random addresses (not my own wallet). I can write the php to do this but wondering if there is a library somewhere. I searched github and didn't see this kind of function.