Even though you posted this in the HTML forum section, I assume that you are talking about using PHP to include these?
A word of caution about putting PHP code (including PHP variable and constant define(...) statements) in files that do not have a .php extension. If you put PHP code/variable/define statements in files that have a .htm, .html, .txt, .dat, .inc, ... or any other extension that is not parsed by the PHP language engine, then anyone that learns or guesses the file name can directly browse (or use a script) to your file and see the exact contents of that file. Only put PHP code/variable/define statements in a .php file (or an alternate extension that you have setup on your server to be parsed as PHP code.)
If you are learning PHP, developing PHP code, or debugging PHP code, do yourself a favor and check your web server log for errors and/or turn on full PHP error reporting in php.ini or in a .htaccess file to get PHP to help you.