Add fields/custom fields to page in the admin page (backend)
Craft CMS is a full-featured CMS. You should not need to use PHP at all unless you need very custom functionality.

It sounds like what you should do is create an "Articles" Section, and add whatever fields you need for articles into that section's layout. Then you'll have a nice user-friendly way to enter content on the backend.

Please see: Sections and Entries