If you're new to Bitcoin, I would advise you to play around btc wallet on testnet you could check mycelium for instance and get some btc via faucet. You need to understand that you don't need any third party for interacting with bitcoins, you just require a wallet. A wallet is a program that handle UTXOs, craft transactions and broadcast them to bitcoind nodes (or other compliant full node implementation), waiting for them to be mine. As you're developper, I highly recommend you to buy Mastering Bitcoin from Andreas M. Antonopoulos.
After than, you will simply design you're backend so that you can accept bitcoin payment, there is many open source project for your needs as btcpayserver or bitcore. I would suggest btcpayserver on your case, but both are "basically" bitcoind wrapper on steroids tailored for enabling applications on top of Bitcoin.
You can write your own php software that does RPC calls to your bitcoin node, see https://bitcoincore.org/en/doc/0.18.0/ for more info
– JBaczuk – 2019-06-06T22:45:02.403