Hi Paul,
I have
<link rel="icon" href="favicon.ico" type="image/x-icon"/>
<link rel="shortcut icon" href="favicon.ico" type="image/x-icon"/>
in both index.php and top.php
It seems the shortcut term is no longer required but typically remains.
However, on checking Ken's original sources I see he has only
<link rel="shortcut icon" href="favicon.ico" type="image/x-icon"/>
in top.php
Just tested and found that everything works as Ken has it (of course it would!!
There is also some useful info here
https://en.wikipedia.org/wiki/FaviconThe .ico file typically will have multiple sizes and colour depths.
The .png format cannot store these multiple file sizes and colour depths which are one of the requirements for an icon file.
However pretty much all modern browsers are set up to search for favicon.ico in the root folder of the website and thus will display the favicon without any link references in the web page(s) themselves.
Have also tested this and find it is correct for IE11, Edge, Chrome and FF.
So maybe the issue is the format of the .ico file.
There are a number of online favicon generators available so may be you could try one of those and see if it makes a difference? You will find these all generate multiple size and colour depth images within the same file.