adding a page in cpanel

I would like to add a custom php page to my wordpress site without using the “add page” option in wp-admin.

At first I thought it would be as simple as adding my page into the same directory as index.php etc in my theme dir in cpanel. When I link to it(
href=”wp-content/themes/mytheme/myphppage.php”)the static content displays but I get an error about functions being undefined. I had assumed that these functions were visible everywhere. How do I include an @import or an include for the functions.php, the wp-includes/category-template.php or whatever files I need to reference?