Include custom template and use dynamic variables?

[Blackshark]

I want to use a custom navbar which is located above the forum.
So i created a custom template which holds the basic HTML structure, lets say i named it 'ownnav'.
I placed '$ownnav' at the top of the original header template.
Then i created a plugin which hooks into 'global_start' and executes '$ownnav = fetch_template('ownnav');' now the HTML stuff of my customtemplate is being executed.
But when i try to use variables inside my custom template they won't get parsed, instead they are being echoed right out.

[Opserty]

Use the following PHP in your hook:
eval('$ownnav = "'. fetch_template('ownnav') .'";');