private var in class

Suppose I have a class: connect_db, and a variable $passwd, which I want to be private in that class. What can I do to prevent someone do something like: $myApp = new connect_db(); echo $myApp->passwd; I'm running Apache server with PHP 3.0.8, and indeed I can do that to access variable in my API classes, which make it really vulnerable. Any suggestion? is this problem fixed in php4? Thanks Reuben D. Budiardja