PhP and mySQL testing - php

Question

I have been trying to test a login php but after i put the username and password i get this error(Notice: Undefined index: username in /web/stud/u1177827/store_admin/admin_login.php on line 12 Notice: Undefined index: password in /web/stud/u1177827/store_admin/admin_login.php on line 13 Faild connection.That information is incorrect, try again Click Here)
here is the code:
<?php
session_start();
if(isset($_SESSION["staff"])){
header("location:index.php");
exit();
}
?>
<?php
// Prase the log in form if the user has filled it out and pressed "Log in"
if(isset($_POST["username"])&& isset($_POST["password"])){
$username=preg_replace('#[^A-Za-z0-0]#i',"",$_SESSION["username"]);//filter everything but numbers and letters
$password=preg_replace('#[^A_Za-z0-9]#i',"",$_SESSION["password"]);//filter everything but numbers and letters
//Connect to the MySQL database
include "../storescripts/connectToMySQL.php";
$sql=mysql_query("SELECT*FROM B4UStaff WHERE fname='$username' AND lname='$password'LIMT 1");//query the person
//..... MAKE SURE PERSON EXISTS IN DATABASE....
$existCount=mysql_num_rows($sql);//count the row nums
if($existCount==1){//evaluate the count
while($row=mysql_fetch_array($sql)){
$staffNo=$row["staffNo"];
}
$_SESSION["staffNo"]=$staffNo;
$_SESSION["username"]=$username;
$_SESSION["password"]=$password;
header("location:index.php");
exit();
} else{
echo 'That information is incorrect, try again Click Here';
exit();
}
}
?>
Does any one know how to fix it?

Answer 1

Correct this two lines instead of SESSION you have to use POST like this
$username=preg_replace('#[^A-Za-z0-0]#i',"",$_POST["username"]);//filter everything but numbers and letters
$password=preg_replace('#[^A_Za-z0-9]#i',"",$_POST["password"]);//filter everything but numbers and letters

Answer 2

Simply change $_SESSION with $_POST:
$_SESSION["username"]
$_SESSION["password"]
to
$_POST["username"]
$_POST["password"]
In your code, you need to get these two POST variables and set it as SESSION variables with staffNo which is correct.