I have a script teller.php, when an unauthorize operation is performed in teller.php I want an alert to pop up in another page i.e account.php about the unauthorized operation is performed.
Is the illegal operation being detected on the page or during server processing? If its on the page you need to have some javascript to detect it (or ajax to check against backend rules) and then give the "alert". If its backend processing then you just need to generate the correct html/javascript to warn the user.
This is the scenerio. if the cashier try to post the amount that is more than the limit set, then the system should alert the system admin automatically. that is a pop-up alert or flash.
Save the alert in a Database from teller.php. Then poll the database for new alerts from account.php Either this, or make a socket server based application for realtime data update
PHP doesn't work that way, popping up on some user's desktop. The user calls the PHP file (by going to its address) and the PHP file does what it was written to do. The user has to start the action, the page can't. What you want is teller.php to send an email or a chat program announcement or something else that can pop up on a user's desktop. Decide what you want to do, and someone can point you to code to do it or write some code for you. (Emailing is trivial - see the manual for the mail() function.)
kanikakaminial, read the post. Person A enters something wrong and he wants person B's computer to pop up a notice.
System Admins tend to be two screen type of people so it's entirely realistic for them to have a browser open purely for system monitoring. That dedicated browser has a page similar to this site's spy that polls and shows new alerts that need to be managed. Each new item that polls could come with a sound file that plays once to attract the system admins attention. All very doable as suggested by The Webby and Rukbat. I guess we need the OP to come back and tell us if more advice is needed.