How do you add outside variables to WP with include()?

I want to use a non-WP file filled with variables from outside the site (just string variables – no database calls) and include it so that if I use any of those variables within a WP template, the variable is executed. When I try to include the file, all variables from that file show up blank. How can I fix this to work and where is the optimal place to include this file once so that all WP files will use this include? Thanks ??