php - Can't seem to get mod_rewrite to set an environment variable

This is just plain weird.

I have put the following in an .htaccess file:

RewriteRule ^a-file-on-the-server$ index.php [E=let_me_in:test]

And in my PHP script, I have the following:

print_r($_ENV);

...which prints out all the environment variables.

When I go to mydomain.com/a-file-on-the-server, I get the output:

Array
(
    [DOCUMENT_ROOT] => ********
    [GATEWAY_INTERFACE] => CGI/1.1
    [HTTP_ACCEPT] => application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5

    [HTTP_ACCEPT_CHARSET] => ISO-8859-1,utf-8;q=0.7,*;q=0.3
    [HTTP_ACCEPT_ENCODING] => gzip,deflate,sdch
    [HTTP_ACCEPT_LANGUAGE] => en-US,en;q=0.8
    [HTTP_CACHE_CONTROL] => max-age=0
    [HTTP_CONNECTION] => keep-alive
    [HTTP_COOKIE] => ********
    [HTTP_HOST] => ********
    [HTTP_USER_AGENT] => Mozilla/5.0 (X11; U; Linux x86_64; en-US) AppleWebKit/533.4 (KHTML, like Gecko) Chrome/5.0.375.99 Safari/533.4
    [PATH] => /bin:/usr/bin
    [QUERY_STRING] => 

    [REDIRECT_STATUS] => 200
    [REMOTE_ADDR] => ********
    [REMOTE_PORT] => 36345
    [REQUEST_METHOD] => GET
    [REQUEST_URI] => ********
    [SCRIPT_FILENAME] => ********
    [SCRIPT_NAME] => ********
    [SERVER_ADDR] => ********
    [SERVER_ADMIN] => ********
    [SERVER_NAME] => ********

    [SERVER_PORT] => 80
    [SERVER_PROTOCOL] => HTTP/1.1
    [SERVER_SIGNATURE] => 
    [SERVER_SOFTWARE] => Apache
    [UNIQUE_ID] => ********
)

As you can see, the environment variable is not showing up. What am I doing wrong?