Environment variables are not passed from .htaccess to PHP

I am trying to pass an environment variable from .htaccess through to PHP. This works just fine on my local WAMP server, but on the server where my website is hosted, it fails without reason.

Here's my test configuration:

.htaccess:

SetEnv TEST_VARIABLE test_value

test.php:

<pre>
getenv('TEST_VARIABLE') = <?php print getenv('TEST_VARIABLE'); ?>

getenv('REDIRECT_TEST_VARIABLE') = <?php print getenv('REDIRECT_TEST_VARIABLE'); ?>
</pre>

On my local server, getting test.php correctly returns:

getenv('TEST_VARIABLE') = test_value
getenv('REDIRECT_TEST_VARIABLE') = 

But on the production server, it returns:

getenv('TEST_VARIABLE') = 
getenv('REDIRECT_TEST_VARIABLE') = 

Things I've ruled out:

1. mod_env is not installed/enabled by the host. Can't be, because then SetEnv would not be recognized and I'd get a 500 while processing the .htaccess.

2. AllowOverrides in httpd.conf for this directory doesn't include FileInfo. Can't be, because then Apache would throw an error "SetEnv not allowed here" when encountering the SetEnv directive and I'd get a 500 again.

3. variables_order in php.ini doesn't include 'E'. This would explain the $_ENV superglobal being empty (which it is), but not why getenv() doesn't return values for these variables.

4. Entire environment is screwed up. Can't be, because getenv('PATH') and getenv('SERVER_NAME') still return valid values.

At this point I'm at a loss as to what configuration could be causing this.