Have a look at this piece of code:

class User {
  var $Name;
  var $Mail

  function SetUser() {
    $this->$Name = "Bill";
    $this->$Mail = "bill@whitehouse.gov";
    echo $this->$Name, ":", $this->$Mail;
  }
}

Maybe you will notice the mistake with the $ sign ($this->$Name instead of $this->Name, see bug report #60). But PHP doesn't print an error, it interprets the code and outputs this:
bill@whitehouse.gov:bill@whitehouse.gov
So, either PHP should produce an error, or there should be a better description of the missing $ sign in the "Classes and Objects" section of the documentation.