Putting a variable in a URL

Scenario:
A file is needing to be referenced in several places.
The name of this file changes often.
The folder where this file is remains the same, so I just want to alter the filename.

So the link would normally be:
<a href="http://very/long/path/filename.txt">

This makes for messy editing and increased chance of mistakes. So…. I want to include a file with the name in and then include that in the url as a constant:

<?php include(‘constants.php’); ?>
and that has
$file1=rabbits.txt
$file2=carrots.txt

Then in the main code body
<a href="http://very/long/path/<?php $file1 ?>">

Unsuprisingly (because I’m writing it) this does not work and when I view source I see
http://very/long/path/

How do I do this ?

Thanks 🙂