php图片写入中文乱码怎么办

$im = imagecreatetruecolor(400,40);    
$black = imagecolorallocate($im,0,0,0);    
$white = imagecolorallocate($im,255,255,255);    
$grey = imagecolorallocate($im,128,128,128);    
imagefilledrectangle($im,0,0,399,39,$white);    
$text = mb_convert_encoding("呵呵呵","UTF-8","GBK");    
$font = "simsun.ttc";    
imagettftext($im,20,0,12,21,$grey,$font,$text);//输出灰色字符串作为阴影    
imagettftext($im,20,0,10,20,$black,$font,$text);//输出黑色字符串作为文本    
header("Content-type:image/png");    
imagepng($im);    
imagedestroy($im);    
?>