I quickly Googled 'codeigniter web services' and it doesn't refer to anything palpable.
Codeigniter on the other hand seems to be a development framework. I'm not going to dig into it, but the only thing I can tell you is that the OpenMRS Platform ships with a pretty exhaustive REST API.
I don't know from where you are going to reach the REST API, but as long as you are doing it from a place that lets you do it, it will work.
The only thing I can tell you with the information that you provide is: yes, OpenMRS provides a RESTful API.