php - Undefined index with $_POST

I am trying to relearn some PHP basics for making a simple login
script, however I get an error I have not received before(I made the same script a
little over a year ago and never had this error. I simplified the code as much as I
could to test to see which area was problematic and here is the
issue:

$user =
            $_POST["username"];
if($user != null)
{
 echo
            $user;
 echo " is your
            username";

}
else
{
 echo "no
            username
            supplied";
}
?>

Now
this code works fine when I send a variable to the script, but when no variable is
supplied it spits out an error. In theory this will be fine because if no username/pass
is supplied then an error is expected. I will be checking to make sure of this before
the code is send to the script, however I fear that somehow a blank string may leak
through and spit out some unknown error. Here is the error I
get:

( ! ) Notice:
            Undefined index: username in C:\wamp\www\verify_login.php on line
            2

Call Stack

 Time Memory Function
            Location
1 0.0003 668576 {main}( )
            ..\verify_login.php:0

no
username supplied

as you can see the
code registers that no variable was supplied, but it gives out and error that I assume
means that a variable was not found were one was expected or something like that. Can
someone please clarify this for me?

class="post-text" itemprop="text">

class="normal">Answer

In PHP, a
variable or array element which has never been set is different from one whose value is
null; attempting to access such an unset
value is a runtime error.

That's what you're
running into: the array $_POST does not have any element at the
key "username", so the interpreter aborts your program before
it ever gets to the nullity test.

Fortunately,
you can test for the existence of a variable or array element without actually trying to
access it; that's what the special operator isset
does:

if
            (isset($_POST["username"]))
{

 $user =
            $_POST["username"];
 echo $user;
 echo " is your
            username";
} 
else 
{
 $user = null;

            echo "no username
            supplied";
}

This
looks like it will blow up in exactly the same way as your code, when PHP tries to get
the value of $_POST["username"] to pass as an argument to the
function isset(). However, isset() is
not really a function at all, but special syntax recognized before the evaluation stage,
so the PHP interpreter checks for the existence of the value without actually trying to
retrieve it.

It's also worth mentioning that as
runtime errors go, a missing array element is considered a minor one (assigned the
E_NOTICE level). If you change the
error_reporting level so that notices are ignored, your
original code will actually work as written, with the attempted array access returning
null. But that's considered bad practice, especially for
production code.

Side note: PHP does string
interpolation, so the echo statements in the
if block can be combined into
one:

echo "$user is your
            username";