Check if user with ID or name X is online

Is there any wa开发者_开发知识库y to check if a user with a certain id or name is online in Wordpress?

I think the function you want is is_user_logged_in. Maybe try

wp_set_current_user($id, $name = '')

To set the current user by id, and then check if they are logged in using

is_user_logged_in()

http://codex.wordpress.org/Function_Reference/wp_set_current_user

http://codex.wordpress.org/Function_Reference/is_user_logged_in

jkeesh has a good solution, but that may not offer live information. is_user_logged_in() is a conditional statement for performing actions.

It's helpful if you want to do this for example:

When logged in, show smiley.jpg

When logged out, don't show smiley.jpg

If you are simply trying to do one of those features which forums have that show "x number of users online" and list their usernames, I don't believe this conditional statement can do that. That would either require some custom code or another plugin.