Hello Pavan,
The redirect_uri_mismatch error usually occurs when the wrong type of credentials used in the program and the redirect_uri is not matching with the one specified in the API console for web application type. You could refer
this guide to create the client ID and client Secret for Web flow and update the redirect URI. Please give it a try and let me know if you are still facing the issue.
Our client library codes are meant to run from command line but not as web pages. Since, PHP Codeigniter is a third party application you have to find a way to call the PHP programs from that software and use the out come results.
Regards,
Sai Teja, AdWords API Team