Kalkun is an open source web-based SMS (Short Message Service) manager.
It uses Gammu SMSD (part of gammu family) as SMS gateway engine to deliver and retrieve messages from your phone/modem.
Documentation is on the wiki: https://github.com/kalkun-sms/Kalkun/wiki
Development and source code can be found on github: https://github.com/kalkun-sms/Kalkun/
Send Bulk OTP SMS in PHP using SMS Gateway Center HTTP API
You need to be registered member with https://www.smsgatewaycenter.com/ and then you can use this script to send OTP SMS to your clients and peers.
This script is just an example to send sms and validate OTP using session. But you can use it with your database to get users and send out OTP SMS to registered members and have them validated.