> using functions as tag libraries.

Care to post an example ?

Basically you split the PHP code into three layers, trying to do a kind of bare bones MVC.

You have the PHP business logic code as usual.

Then instead of doing

    <a href="http://somewhere">A link</a>

You have a view function that does that for you

    function web_link($url, $title) {
        echo "<a href="$url">$title</a>";
    }

And then on the HTML markup, you place those calls as tag libraries in other similar frameworks,

    web_link("http://somewhere", "A link")  

Very low tech, but does the job similar to JSP tag libraries and provides some organization.

It seems too low tech to be useful for any project of any complexity. Existing HTML template frameworks do a lot of things like character and syntax aware data escaping (and avoiding double-escaping), template caching, plugins, code generation, tag extension, filters, etc.

Other than being slightly more concise and appearing more functional, I don't see the benefit of this over something like Twig or Smarty. At some point you're going to wind up re implementing features that already exist in a framework anyway.

Naturally if one is doing it at scale, this is NIH.

I used it only for my web site, and many alternatives did not exist back when I did this.

Yes that is the idea, it just did not exist in 2003.

Low-tech is fine, however I'll assume your example has been excessively simplified for the sake of brievity because as-is it's prone to break your page.

Yes, I was making it basic, just describe the idea behind it.