Check whether $_POST-value is empty

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

I cannot get the $username to be “Anonymous”? It is either blank or the value of $_POST['userName'].

Answer

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';
}