I have a navigational bar, an image, and a heading that I\'ll be including in every page of my website, so I wanted to use php include to refer to this code in several pages
You can use php code in files with extension .php and only there (iff other is not defined in your server settings).
Just rename your file *.html to *.php
If you want to allow php code processing in files of different format, you have two options to do that:
1) Modifying httpd.conf to allow this for all projects on your server, by adding:
AddHandler application/x-httpd-php .htm .html
2) Creating .htaccess file in your separate project top directory with:
AddType application/x-httpd-php .html
For second option you need to allow use of .htaccess files in your httpd.conf, by adding the following settings:
AllowOverride All
AccessFileName .htaccess
*that is correct for Apache HTTP Server