External file import question

[Vcize]

I am attempting to include a navigation bar at the top of my site and have followed some of the guides to do so using an external php script. However, the only way I've been able to figure out to do it is by creating the plugin and then including the plugin in each of my templates via a variable.

For instance...

Plugin code

ob_start();
  require_once('foo/foo.php');
  $foo_include = ob_get_clean();
ob_end_clean();

That runs on global_start.

Then, in my template I call $foo_include and that imports my foo.php file.

Is there a way I can hook into the templates to automatically include all the plugins at the very beginning of each template without the user having to add the variable manually? It needs to be the very first thing run, before any other elements are rendered.