Hi Malcolm,
I just tried out your example, with one change in the main program::
<html>
<body>
the cat sat on the mat aa a a a a a a a a a a a a a a <br>
before <br>
=============================== <br>
<? include "xxx.html" ?>
=============================== <br>
after <br>
the cat sat on the mat zz zz z z z z z z z zz z z z z z <br>
</body>
</html>
and without the html start and end tags in the xxx.html file
(even if you call the file xxx.php, the contents will always be interpreted as HTML unless you use the <? and ?> PHP tags.)
now is the time for all good men to come to the aid of the party a a a a a a <br>
this is the inserted document <br>
now is the time for all good men to come to the aid of the party z z z z z z z <br>
the
br-Tags are just line feeds, to format the text.
You need to load it to your website, as normal home computers don't have PHP, the RootsChat free webspace does

You can see the results here:
http://tutorial.rootschat.net/test.phpRe-reading your reply to Stonechat:
with 1100 linked pages I am wondering whether it might be an idea for you to download a (free) PHP environment like XAMPP, which gives you Apache, MySQL and PHP, so that you can test your pages offline first.
Bob