试试在CCaptchaAction.php的renderImageGD($code)的末尾部分，输出图像之前，加ob_clean()清空输出缓冲区：

protected function renderImageGD($code)
{
         。。。。。。  //省略xxx行
	imagecolordeallocate($image,$foreColor);
	ob_clean();  // <================================ 情况输出缓冲区
	header('Pragma: public');
	header('Expires: 0');
	header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
	header('Content-Transfer-Encoding: binary');
	header("Content-Type: image/png");
	imagepng($image);
	imagedestroy($image);
}