Can I count sessions to determine number of people online?

Question

I am confused about something. When I try to search how to count online users in PHP, all answers related with MySQL and many different ways.

In my script, any user that submits the login form creates a $_SESSION['$nickname']

So I thought, can I count login sessions with count($_SESSION['$nickname']); and show it in my page?

Or is this totally a wrong logic?

Solution 1

Totally wrong logic. $_SESSION is a per-user thing. One user's session is not shared with any other user's session. Think about it - an online bank written in PHP, all sharing a single $_SESSION - everyone would see everyone's account details.

Assuming you're on the standard PHP file-based sessions, you can count the session files in whatever directory they're stored, e.g.

$users = count(glob(session_save_path() . '/*'));

Note that this just counts session files - it will undoubtedly contain stale/dead sessions that haven't been garbage collected yet. If you want an actual "really is online right now", you'd have to parse each session file and examin its contents.

Solution 2

At first, you have to define what "to be online" means. Should the user have clicked on a link within the last 5 minutes?

I assume that you already have a user table in your database. So the simplest way is to add a new column, e.g. lastAction TIMESTAMP.

And when the user clicks on a link on your page, your script should update this value.

And on your statistics page or whatever, you get the number of online users with that code:

SELECT COUNT(*) FROM users WHERE lastAction > (NOW() - 60*5)

Solution 3

PHP is pretty flexible in terms of session storage, you can define your own session save/restore handlers.

However, the default session storage is files, where each session is individually stored to disk. Which means, that in order to find out how many users there are "online" (here, i assume "online = session exists with $_SESSION['$nickname'] set"), you would need to open all the session files stored on disk, and check how many unique nicknames exist within them. This is very heavy in both time and required resources.

Hence, most tutorials suggest counting this in the database, by maintaining a last-seen timestamp per user (and checking how many users were last seen in last X minutes).

If you wish to combine, that is doable via defining your own session save handler to store session information in the database...

---

Several additional notes on sessions:

• Sessions are not destroyed immediately when closing a browser. In fact, the browser does not tell the server in any way that it is being closed. It means that the server should come up with some time-based algorithm to decide who's online and who is not.
• Sessions are isolated from one another so $_SESSION["foo"] can not be shared by multiple different sessions - there is no equivalent to Global.asa in PHP. Not off the box.

Comments

[Slavez]

Yes, this is good information but it will create too many MySQL connections and updates. I just want count users that created a session. We know if they shut down their browser or close my site, session will be destroyed.

[ComFreek]

If you only want to count the current number of session (files), Marc B has already posted your solution.

[Slavez]

This was very helpful and I am going to do some changes with this information, thank you.

[Slavez]

I really loved lastaction way of counting online users, by the way. Does it tire the server? Cause, every page will make an update, you know.

[poncha]

There are two problems here: (1) not every session might have a nickname in it, and (2) those nicknames are not necessarily unique...

[poncha]

Why INTEGER btw? you compare it to time, so why not TIMESTAMP ?

[Slavez]

"This is the wrong logic and the session is only stored locally on the end user's PC" this is true information for Cookies not for Sessions.

[poncha]

oh, and about garbage-collected files... you can use find (execute) to find how many session files were modified in last N minutes...

[ComFreek]

@Slavez That depends on your server and on the number of users you have. But it isn't as expensive as you think.

[Delorean]

True, but I guess I should clarify that sessions are destroyed (locally) immediately when a browser is closed, but not on the server. The server can retain it for much longer.

[Cups]

You might investigate how to store sessions for this app in a different directory, also set a shorter life for them (but be careful -- not too short) then do the equivalent of "-latr" and count just those created in the last x minutes.

[Sablefoste]

@Slavez, I don't think you can determine if a user shut down their browser; consider if the user closes a tab, and the "reopens the last tab" with a ctrl-shift-t. If the $_SESSION hasn't expired, it should reopen just find. Unless you are constantly pinging the server and have a very short expiration time for the $_SESSION.