How create page with PHP/SQL functions?

I need to create a page that displays and updates information from a new SQL table and have it show up as one of the items on the menu.

My PHP is rusty, but sufficient to create a stand-alone .php file that performs the necessary functions.

Inserting PHP code into a WordPress page or using the page redirect at the bottom of the edit form do not seem to work.

How do I set up a link in the menu to a stand-alone .php file or, better yet, create a WordPress page that runs the PHP functions and HTML code?