Well, PHP SMS gateway integration is a simple procedure. It doesn’t take too much time. In general, we must pass some parameters to their call back URL of API to send messages. Usually, these gateways provide 3 types of plan 1) One-way communications 2) Two-way communications 3) Both ways communications. Here one way only allows dispatch messages to the customer, but two ways also allow to receive reply messages. Based on the OTP SMS Gateway provider, API may quite from each other.