How to Create a Login system in PHP Using a Session

In this article, we will explain how to create a login system in PHP using a session.

['username']!=$username && $row['password']!=$password)
{
$error="<p>Wrong User Name and Password</p>";
}
if(isset($error))
{
echo $error;
}
else{
require('db.php');
session_start();
$result =mysql_query("select * from users where username='$username' and password='$password'");
while($row=mysql_fetch_array($result))
{
$_SESSION['username']=$row['username'];
echo"<script>window.location.href='../invoice/'</script>";
}
}
}
?>

db.php

<?php
mysql_connect('localhost','root','');
mysql_select_db('testbill');
?>