<?
$projectname = $_POST[projectname];
class Thumbnailer {
    var $autocreate = true;
    var $single_mode = false;
    var $image_list = array();
    var $ersetzen = false;
    var $image_ext = array("jpg");
    var $image_dir = "$projectname/original/"; //hier ist der Fehler
    var $thumb_dir = "";
    var $thumb_w = 300;
    var $thumb_h = 350;
...

class Thumbnailer { 
    var $autocreate = true; 
    var $single_mode = false; 
    var $image_list = array(); 
    var $ersetzen = false; 
    var $image_ext = array("jpg"); 
    var $image_dir = ""; 
    var $thumb_dir = ""; 
    var $thumb_w = 300; 
    var $thumb_h = 350; 

## nur einen von beiden!

    function __construct() # Constructor PHP 5
    {
         $this->varset();
    }

    function Thumbnailer() # Constructor PHP 4
    {
         $this->varset();
    }

##########################

    function varset()
    {
         $this->image_dir = $_POST[projectname] . '/original/'; #ä oder was Du willst...
    } 
}