Post Username or not

[NZ_Kiwis]

I want to check if the user has entered a username or not, this code does not work, why and what should i have?

if (!isset($_POST['username'])){
echo "You have not entered a username<br>";
$err = 1;
}

[dagon]

empty()

[bradgrafelman]

In other words, simply not entering a username won't make [man]isset/man return false, since the form element will still be set (an empty/null value is a perfectly acceptable/settable value).