验证码乱码有关问题

验证码乱码问题
我写了一个验证码类，为了大家帮忙找错误，我把类的代码贴出来，编码格式gbk

header ("Content-type: image/png; charset=gbk");
class CheckCode {
private $mCheckCodeNum = 4;
private $mCheckCode = '';
private $mCheckImage = '';
private $mDisturbColor = '';
private $mCheckImageWidth = '80';
private $mCheckImageHeight = '20';
public function CreateCheckCode()
{
  $this->mCheckCode = strtoupper(substr(md5(rand()),0,$this->mCheckCodeNum));
  return $this->mCheckCode;
}

private function CreateImage()
{
  $this->mCheckImage = @imagecreate ($this->mCheckImageWidth,$this->mCheckImageHeight);
  imagecolorallocate ($this->mCheckImage, 200, 200, 200);
  return $this->mCheckImage;
}

private function SetDisturbColor()
{
  for ($i=0;$i   {
  $this->mDisturbColor = imagecolorallocate ($this->mCheckImage, rand(0,255), rand(0,255), rand(0,255));
  imagesetpixel($this->mCheckImage,rand(2,128),rand(2,38),$this->mDisturbColor);
  }
}

public function SetCheckImageWH($width,$height)
{
  if($width==''||$height=='')return false;
  $this->mCheckImageWidth = $width;
  $this->mCheckImageHeight = $height;
  return true;
}

private function WriteCheckCodeToImage()
{
  for ($i=0;$imCheckCodeNum;$i++)
  {
  $bg_color = imagecolorallocate ($this->mCheckImage, rand(0,255), rand(0,128), rand(0,255));
  $x = floor($this->mCheckImageWidth/$this->mCheckCodeNum)*$i;
  $y = rand(0,$this->mCheckImageHeight-15);
  imagechar ($this->mCheckImage, 5, $x, $y, $this->mCheckCode[$i], $bg_color);
  }
}

public function OutCheckImage()
{
  $this ->CreateCheckCode();
  $this ->CreateImage();
  $this ->SetDisturbColor();
  $this ->WriteCheckCodeToImage();
  imagepng($this->mCheckImage);
  imagedestroy($this->mCheckImage);
}
}
?>

在此页面内 定义输出
$c_check_code_image = new CheckCode();
$c_check_code_image ->OutCheckImage();
都完全正常，正常显示一个验证码
但是我新建一个PHP页面，把include这个类之后，放上上面两句代码就出现乱码了，错误代码提示Warning: Cannot modify header information - headers already sent by (output started at E:\www\HSreport\check_code.php:1) in E:\www\HSreport\class\CheckCode.class.php on line 2
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