how to send a mail to users with their password at a time

have you attempted it on your own already? post your code...

No

For mailer, you can use PHPMailer

Nobody will just give you the solution. You will at least need to attempt it by yourself first. Google has many tutorials out there. Learning something for yourself that you need to acheive is the best way to learn.

If then you are stuck let us know what exactly you are having trouble with.

You can do this just using PHP and SQL but no one will just give you the code.

I can recommend this link: http://www.htmlgoodies.com/beyond/php/article.php/3855686/PHP-Mailer-Script-Step-by-Step.htm

It takes you through creating a mailer script step by step explaining every step of the way.

You need to connect to your SMTP server and send the emails in a loop. In each loop you are going to need to collect the recipients email address, username and password.

Have fun and good luck!

You can revise this tutorial: http://www.computersneaker.com/php-mail-script/

I think you're looking for something like this (still to be worked out, of course):

<?php
for($i=0; $i<count($records); $i++)
{
    $record = $records[$i];

    // Set data.
    $password = $record['password'];
    // etc...

    // Mail the user his login info.
    mail(...);

    // You're done!
}

@minitauros: Although correct, I don't think most hosts will allow you to send that many emails in such a short period.