Check whether $_POST-value is empty

Solution 1:

isset() will return true if the variable has been initialised. If you have a form field with its name value set to userName, when that form is submitted the value will always be "set", although there may not be any data in it.

Instead, trim() the string and test its length

if("" == trim($_POST['userName'])){
    $username = 'Anonymous';
}      

Solution 2:

If the form was successfully submitted, $_POST['userName'] should always be set, though it may contain an empty string, which is different from not being set at all. Instead check if it is empty()

if (isset($_POST['submit'])) {
    if (empty($_POST['userName'])) {
        $username = 'Anonymous';
    } else { 
        $username = $_POST['userName'];
    }
}