Good, but simplistic, at this point.

Good, but simplistic, at this point. It could be expanded considerably into a more valuable template object. As it sits, it's not too much better than just manipulating key phrases in a string via str_replace(). It does encapsulate into an object, though, and that's always some advantage.
Suggestions: 1) sub templates. Allow templates to be included inside other templates, with recursive processing. 2) variable, context-dependent content. Allow a specifier that can modify the final result that is inserted. Simple example would be date, a better one would be page number, or "this page's" title or other page-specific value, or for language. 3) dB support, for that is where the great power of a template system becomes real.