How can you tell whether a user is still online

PHP
1

how can you tell whether a user is still online?

please excuse me if this isn’t a php ques.

2

In what context? What kind of website are you referring to?

3

…still online? As in has the user not hung up the connection?

or do you mean:

still looking at and interacting with your webpage?

or

still got one of his browser tabs with your webpage loaded?

or something else?

4

You can’t tell in real time unless their browser is currently making a request for information from the server.

The best you can do is to assume that anyone who made a request in the last X minutes is still online. Five or ten minutes are typical time periods used for this by various sites.