<?php
if(isset($_POST['submit'])) {

$username = $_POST['username'];
$password = md5($_POST['password']);

$logincheck = $stmt = $con->prepare("SELECT * FROM users WHERE username=:username and password=:password");
$stmt->bindParam(':username', $username);
$stmt->bindParam(':password', $password);
$stmt->execute();
$rows = $stmt->fetch(PDO::FETCH_NUM);
if($rows > 0) {
exit(header('Location: index.php'));
}
}
?>

<form method="post">
<input type="text" name="username">
<input type="password" name="password">
<input type="submit" name="submit">
</form>