$_phpinclude_output?

[Mazinger]

What does $_phpinclude_output do (in the header)?

[Jake Bunce]

That is like a template hook. The default header template contains that variable so you can simply assign some output to that variable to display it up in the header.

[kentaurus]

Suggestion: rename it to $template_hook[header]

[Jake Bunce]

That is probably a good suggestion. I think it's a left over from the days of 3.0.x when there were still phpinclude templates. Starting in 3.5.x those templates were replaced by the plugin / hook system.