in a Simple CMS sytem, it requires to add new pages. So, what will be the reliable way? create new file using php or save the page contents to database & show retrieving them on a template?
depends on what you want and what the CMS is designed to handle. If its a lot of content, the database solution will be the best for managing. If its a mini site/brochure site, you could just use files.
I would go as far as to say a database is almost always the correct way to go, given that a CMS is usually overkill for a site with few pages (<10) and a database will almost always be needed, both for easy of coding, and an ability to deal with new things (additions) without having to change major parts of the codebase. Go for a database solution, I'd say.