[wvenable]

One could simple write their own "echo" function which only takes an XML object parameter.

    // This works
    write(<span class="username">{$_POST['name']}</span>); 
  
    // Where as this wouldn't work
    write($_POST['name']);

I think perhaps the use of echo in these examples is just a simplification. More than likely, in Facebook, these XML classes are outputted by another function.