Hi there..
I'm personal freelancer and interesting with your project are offering and also already read your project description..
so hopefully could help you to make callback URL API using PHP.. and currently I'm ready and can do this immediately..
I am capable to do the work for you.. cause I've quite a long experience (15+ years) with full stack web development in PHP, Bootstrap, jQuery, API, and also Database Architecture using MySQL, MSSQL, and PostgreSQL.
I'm looking forward to discuss and collaborating with you!
Thank you
Vincent