Finding total number of active sessions

Question

I'm a beginner to PHP and I'm writing some code to my site. I want to get the total number of sessions that is active at that instant. I knew this is some difficult task but possible. How can I do it?

I've googled and some people say that it is possible by counting the total number of temporary session files in the temp directory. But, where is it located?

For greater explanation consider the example of Joomla backend which shows the total number of current visitors and administrators as shown below:

Answer 1

I am improving the answer from @user2067005. You actually need to deduct 2 from the count because scandir reports the . and .. entries as well.

$number_of_users = count(scandir(ini_get("session.save_path"))) - 2;