.htaccess redirect a url to PHP using RewriteRule

Trying to setup .htaccess so I can redirect a url like http://example.com/io/abc123 to http://example.com/io/io.php?id=abc123. But, for some reason, the RewriteRule does not work as I expect.

I want to keep this redirection local without messing up the root .htaccess file, and so I create a new .htaccess file inside the io subdirectory. (Doing the equivalent in the root directory gives the same outcome, so it's not that).

Directory structure:

www
 |
 +--io
    |
    +-- .htaccess
    \-- io.php

.htaccess file:

RewriteEngine on
RewriteRule ^(.*)$ io.php?id=$1

io.php file:

<?php
  echo "Hello World" . "\n";
  echo "REQUEST_URI=[" . $_SERVER['REQUEST_URI'] . "]\n";
  echo "PHP_SELF=[" . $_SERVER['PHP_SELF'] . "]\n";
  echo "QUERY_STRING=[" . $_SERVER['QUERY_STRING'] . "]\n";
  $id = $_GET['id'];
  echo "id='" . $id . "'\n";
  ?>

OUTPUT

Hello World
REQUEST_URI=[/io/abc123]
PHP_SELF=[/io/io.php]
QUERY_STRING=[id=io.php]
id='io.php'

For reasons I don't understand, instead of the expected query_string id=abc123 this results in id=io.php.

It's obviously getting the rewrite correct to the target file io.php, but fails to substitute the abc123 into the id parameter using $1.

What am I doing wrong?