why am I getting this error "Undefined index:" [duplicate]

This question already has an answer here:

• “Notice: Undefined variable”, “Notice: Undefined index”, and “Notice: Undefined offset” using PHP 28 answers

Why am I getting this Error "Undefined index:"... Everything is working good except "userfirstname" & "userlastname"...

this is the PHP code

$firstname = $_POST["userfirstname"];
$lastname = $_POST["userlastname"];
$email = $_POST["useremail"];
$password = md5($_POST["userpass"]);
$id = 'user'. rand(11111,99999);
$forUserEmail = mysqli_query($con, "Select * from register where 
user_email='$user_email'");
$countRecord = mysqli_num_rows($forUserEmail);