How to change the password

Hello,

I have a log-in page where the user name and password entered is checked with the values in a table and if they match, the user would be re-directed to the page i mentioned in the header.Thatz fine.Now, i would like to change the password through a form.For ex:

I have a form with 3 text boxes to enter the old password(i.e the user which he entered to log-in), to enter a new password and to re-enter the new password.If the entered new password and the re-entered password matches they are stored in the table in encrypted form.If they are not matched, i would like to display an error message.I would like to store this newly entered password value on my old password, that is just over-writing the exsisting one.How can i do this one?
All your suggestions would be highly appreciated.

thanking you in advance.


The following is the piece of code which i'm using for my log-in page.


<?php

Database connection;

$sql = "SELECT count(*) as cnt from TBL_USER WHERE USER_NAME = '$user' AND USER_PASSWORD = '$pass'";
$result = ibase_query($sql);
$row = ibase_fetch_row($result);
if ($row[0]) {
header('Location: main.php');
exit;
} else {
echo"<form action="$PHP_SELF" method="POST">n";

}
// Display Log-in Form
?>

u can use the same code to check if old pass was right, and to actualy change a pass to a newpass, try with a query like: