Notice: undefined index: email in C:\xampp\htdocs\finale\register.php on line 22

Notice: Undefined index: email in C:\xampp\htdocs\finale\register.php on line 22

<?php
mysql_connect('localhost','root','') or  die('cant connect database ');
mysql_select_db("forumseries") or die('cannot select database');
?> 

<form action="register.php" method="post">
username:<input type="text" name="username"><br>

email:<input type="text" name="email"><br>

password:<input type="text" name="password"><br>
<input type="submit"name="submit">
  </form>

<?php
include_once('connect.php');
if(isset($_POST['submit'])){
    $username=$_POST['username'];
    $email=$_POST['email'];
    $password=$_POST['password'];
    if(!empty($username)&&!empty($email)&&!empty($password)){
        $sql=mysql_query("select * from users");
        while ($row=mysql_fetch_array($sql)) {
            $user=$row['username'];
            $eml=$row['email'];
            if($user!=$username&&$eml!=$email){
                $sql2=mysql_query("insert into users(id,username,email,password)values('$username','$email','$password')");
            }
        }
    }else{
        echo"All fields required";   }
}
?>