If you mean the Linux Kernel version then uname -r is what you want.
I've just tired apache_get_version(); on my server and it seems to work correctly for me with the Server API as Apache (ie a module), however, I think what you get returned from that is the same as you'll get from getenv('SERVER_SOFTWARE'); as both return Apache for me as I've set it to not show any other details in httpd.conf.
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