GD2绘制图形在浏览器中不能展示

$im = imagecreatetruecolor(50,25);

$red = imagecolorallocate($im,255,0,0);
$str = 'ABCDEFGHIJKMNPQRSTUVWXTZabcdefghijkmnpqrstuvwxtz23456789';
$str = substr(str_shuffle($str),0,4);
imagestring($im,5,8,5,$str,$red);

header('content-type:image/png');
imagepng($im);
imagedestroy($im);


?>