If you want to use own php file in WordPress, you need to make the file friendly to WordPress so that I can read rest of the core WordPress funtctions. Add this snippet on the top of the php page you create.
Code Snippet To Add and Use Own PHP File in WordPress In Plugins or Themes
About Mohammad Shoeb
Hi, I'm Shoeb. I've been fortunate to work with WordPress for over a decade and I'm continuously learning from this journey. Alongside some wonderful people, I help run Elicus in Jaipur. Our aim is to make WordPress tools that are simple and helpful.