How to use $theme.params.myVar in {php} block.

I would like to use the value of $theme.params.myVar inside a {php} block in my theme.tpl. Is that possible? My specific case is as follows:

In my theme.tpl, I use {php} include("/home/blah/blah/header.php"); {/php} to use my global site header in my gallery. This is working fine, however I'd like to customize my theme a bit and allow the header.php location to be specified in the theme settings. So, I added a new setting to my theme called headerPhpFile which contains the path to my common header.php for my site. After putting my gallery in debug mode I can see that the value is available, but I do not know how to reference it inside the smarty {php} tags. Any suggestions?

Thanks!