Classes and access to outside variables

Hi there, I am new to PHP classes and I do want to access a variable outside the class, but somehow that does not work. global also does not have any effect. In the following example I would like to be able to access $dbh from inside the class like I did in that example. This does not work. Can somebody please give me a hint on the right syntax? $dbh = 'test'; class search_helper extends AjaxACApplication

    {
        /**
         * Database connection details
         */

        var $db_hostname = $dbh;

Thank you for any help, Merlin