Php, mysql and codeigniter

Folks, we are using an application built on Codeigniter with PHP aad mysql. It is working fine and moving on to the final stage. The final stage being,  connecting to our main source, the backend, via rest api. Ultimately,  we would not have mysql. We will connect using rest api to backend. W already have the schema built up fr the services by mimicking the mysql tables. So the question,  can this be done without a mysql db? The producion will not host or support mysql. So we want to use php app talk to the backend via rest api. Any help, appreciated..