How can I tell which user Apache is running as using PHP?

Create a file in your root web directory named whoami.php with the following code inside:

<?php
echo shell_exec('whoami');

Then, access whoami.php using your browser at an address such as:

http://YOURdomain.com/whoami.php

The name of the user Apache is running as will be displayed in your browser.

When you're finished testing, delete whoami.php.