The best way to notify a user as soon as there is some update or event in Web application?

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PHP

web_development

Could you please let me know the best way to achieve this thing, for example when a user is seeing a web page (like a e-mail service), if there is some event or update (like a message received from someone), this should be notified to the user right at that time (nearly real time)?

I've thought of using AJAX and a timer-like handler to check for the update periodically but this seems not to be good for the server (there may be a chance when many users will be visiting the page simultaneously and each user has a loop for checking the new contents in the database which is stored on the server-side).

Is there any other technique?

Your help would be highly appreciated!
Thanks!