Detecting online users: PHP

I am trying to make a thing where it says "X number 开发者_C百科of users are currently viewing this page" via PHP. How would I do this? Thank you.

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There's no concept of "currently viewing" on the web, because everything happens as non-persistent connections. The closest you can get is "X users viewed this page during the last N seconds" (and if you choose a value of N that's small enough, you could say that it's pretty much equivalent).

Then, to have user A view data based on user B's actions, in PHP, you need to have some persistent storage, usually as a database like MySQL (but memcached or xcache could work as well, if you don't really worry about losing the data if the server crashes).

Simply write items such as "user X viewed page Y at time Z" to your persistent storage, and when page Y is displayed, query the storage for all views of page Y in the last N seconds, and count the number of different users.

In SQL:

INSERT INTO visits (user, page, time) VALUES (?,?,NOW())

SELECT COUNT(DISTINCT user) FROM visits 
WHERE page = ? AND time < NOW() - INTERVAL 10 SECONDS

You could have the user tell the server "I'm still viewing" by submitting an AJAX request regularly (or having a small iframe on your page that reloads itself every few seconds without JavaScript).

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1. Create a database table that stores:
   • A hash of user IP and Useragent combination
   • Timestamp
2. Upon each pageload, store hash of user IP and Useragent combination along with current timestamp into database
3. Query database for number of unique entries that have timestamp that falls into NOW() - X minutes range
4. Display approximate number of active users

Expand on the basic idea by

1. Having an interval spawned AJAX call to a script that updates db to account for users staying on a page for longer than X minutes

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There is no failsafe way of doing this, but there are a couple of options.

1. Save the last time the person visited in the database. If it's within a certain threshold (say 5 minutes), they're online.
2. This way is less reliable, but occasionally ping ( correct term? ) the server with AJAX to tell it you're still online. This allows the user to have a page up for a long amount of time without their "online"-ness being timed out.

You could even use a combination of the two.

If you have a lot of traffic, you should cache the first option. You wouldn't want to run SELECT * FROM users WHERE lastvisit > UNIX_TIMESTAMP() - 18000 for every page load. Another option would be to run a daemon for that every minute or so, and store the online users in a separate table.

Note: this assumes you really meant users when you said it. If you want the number of guests online, see @code_burgar's answer.

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You really can't because you have no idea when someone has stopped viewing a page.

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Create a session variable with time:

$_SESSION['time']=time();
$time=time();
$query="INSERT INTO `online`(`name`, `time`)VALUES ('$name','$time')";

**// insert the time and username into the DB..**
if(time() - $_SESSION['time'] >=60)

**//check for ths condition if the user is available do the same thing again**
$_SESSION['time']=time();
$time=time();

$query="INSERT INTO `online`(`name`, `time`)VALUES ('$name','$time')";

While retrieving the online users:

$t=time();

// this will help u to remove the records that are not useful for u.
// **thus helping you to use less space on your database**
mysql_query("DELETE from online WHERE '$t'-time >60");

Finally, perform: mysql_query("SELECT * FROM online");