Simple Login with Session

File index.php :

<?php
include "config.php";
?>

<form action="submit.php" method="post">
<label>Username</label>
<input type="text" name="username">
<label>Password</label>
<input type="password" name="password">
<input type="submit" name="submit" value="Submit">
</form>

File config.php:

<?php
session_start();
$host = "localhost";
$username = "root";
$password = "";
$database = "loading";

$con = new mysqli($host, $username, $password, $database);
if($con->connect_error){
die("Connection failed:" . $con->connect_error);
}
?>

File submit.php:

<?php
include "config.php";
$username = $_POST['username'];
$password = $_POST['password'];
if(isset($_POST['submit'])){
$sql = "SELECT * FROM userlogin WHERE username = '".$username."' AND password = '".$password."'";
$result = $con->query($sql);
if($result->num_rows == 1){
   $_SESSION['username'] = $username;
   $_SESSION['password'] = $password;
    header("location: loginform.php");
}else{
echo "credentials not matched";
}
}
?>

File loginform.php:

<?php

include 'config.php';

$u = $_SESSION['username'];

echo "Welcome " . $u;

if(isset($_POST['logout_submit'])){

session_destroy();

header('location: index.php');

}

?>

<form action="" method="post">

<input type="submit" name="logout_submit" value="Logout">

</form>

File authentic.php:

<?php

include 'config.php';

$username = $_SESSION['username'];

$password = $_SESSION['password'];

echo "Cridential are - <br/>Username: " . $username . " <br/> Password: " . $password;

if($_SESSION['username'] == '' AND $_SESSION['password'] == ''){

header('location: index.php');

}

if(isset($_POST['logout_submit'])){

session_destroy();

header('location: index.php');

}

?>

<form action="" method="post">

<input type="submit" name="logout_submit" value="Logout">

</form>