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 Marshall
Marshall is creative head at Elicus and works with the team to bring ideas to life. He is strongly focused on delivering a quality experience to clients and customers.