add the following in includes/page_header.php near the top
PHP Code:
ob_start();
include("path to php file");
$myvariable = ob_get_contents();
ob_end_clean();
In the same file at the end of template->assignvars list (around line 420 or so) add:
PHP Code:
"MYVARIABLE" => $myvariable
In your template file add the following where you want your code: