Hello
I require a solution please
the problem is the
user register for an event at the admin side the admin can accept or reject is registration for event
if admin clicked on accept a mail has to go saying that the registration has accepted else no mail
please help me

Hi.

Check out PHPMailer.
(See an example of how to use it here)

And there is always the old mail function, if you need something simpler.

If you are asking for a whole admin setup your opening up a can of worms that is a very time consuming and complex task else check out Atlis link to PHPMailer to send mail via php.

OmniX has a point if you do not have this knowledge yourself as of now, then you are looking at spending months trying to build a system like this (with ALLOT of help from other developers), or just hire a freelancer and have it done in a week (if not less). The only disadvantage of the second option is that you will not be learning anything.