PHP生成条形码有关问题

function barcode($listing)
    {
    // Including all required classes
        require_once('class/BCGFontFile.php');
        require_once('class/BCGColor.php');
        require_once('class/BCGDrawing.php');
        
        // Including the barcode technology
        require_once('class/BCGcode39.barcode.php');
        
        // Loading Font
        $font = new BCGFontFile('./class/font/Arial.ttf', 18);
        
        // The arguments are R, G, B for color.
        $color_black = new BCGColor(0, 0, 0);
        $color_white = new BCGColor(255, 255, 255);
        
        $drawException = null;
        try {
            $code = new BCGcode39();
            $code->setScale(2); // Resolution
            $code->setThickness(30); // Thickness
            $code->setForegroundColor($color_black); // Color of bars
            $code->setBackgroundColor($color_white); // Color of spaces
            $code->setFont($font); // Font (or 0)
            $code->parse($listing); // Text
        } catch(Exception $exception) {
            $drawException = $exception;
        }
        
        /* Here is the list of the arguments
        1 - Filename (empty : display on screen)
        2 - Background color */
        $drawing = new BCGDrawing('', $color_white);
        if($drawException) {
            $drawing->drawException($drawException);
        } else {
            $drawing->setBarcode($code);
            $drawing->draw();
        }
        
        // Header that says it is an image (remove it if you save the barcode to a file)
        header('Content-Type: image/png');
        
        // Draw (or save) the image into PNG format.
        $drawing->finish(BCGDrawing::IMG_FORMAT_PNG);
     }