PHP image scaling implementation method_PHP tutorial

    /**
    * image zoom.
   */
    function imageZoom($filename, $w, $h) {
        /* Arguments meaning */
        /* $filename: the source of the name */
        /* $w: you want get the image's width */
        /* $h: you want get the imgage's height */
        $arr = getimagesize($filename);
        $src_w = $arr[0];
        $src_h = $arr[1];
        $src_t = $arr[2];
        /*1 = GIF，2 = JPG，3 = PNG，4 = SWF，5 = PSD，6 = BMP，7 = TIFF(intel byte order)，
= TIFF(motorola byte order)，9 = JPC，10 = JP2，11 = JPX，12 = JB2，13 = SWC，
= IFF，15 = WBMP，16 = XBM*/
        $src_m = $arr['mime'];

        $src_img = imagecreatefromjpeg($filename);

        if (($w / $src_w) >($h / $src_h)) {
            $bili = $h / $src_h;
        } else {
            $bili = $w / $src_h;
        }
        $dst_w = $src_w * $bili;
        $dst_h = $src_h * $bili;
        $dst_img = imagecreatetruecolor($dst_w, $dst_h);

        imagecopyresampled($dst_img, $src_img, 0, 0, 0, 0, $dst_w, $dst_h, $src_w, $src_h);

        header("content-type:{$src_m}");

        switch ($src_t) {
            case 1:
                $imgout = "imagegif";
                break;
            case 2:
                $imgout = "imagejpeg";
                break;
            case 3:
                $imgout = "imagepng";
                break;
            default:
                echo "The type was wrong!";
                break;
        }

        $dst_filename = "s_".$filename;
        $imgout($dst_img, $dst_filename);

        imagedestroy($dst_img);
    }
    $filename = 'gg.jpg';
    imageZoom($filename, 100, 200);