php imagettftext和阿拉伯语编码

Question

我使用此代码在图像上写文本但用阿拉伯语写，问题是字母是从左到右书写的。我用这个代码

header('Content-Type: image/png');
$im = imagecreatetruecolor(400, 30);
$white = imagecolorallocate($im, 255, 255, 255);
$grey = imagecolorallocate($im, 128, 128, 128);
$black = imagecolorallocate($im, 0, 0, 0);
imagefilledrectangle($im, 0, 0, 399, 29, $white);
$text = 'نص عربي';
$text = mb_convert_encoding($text, 'HTML-ENTITIES',"UTF-8");
$text = html_entity_decode($text,ENT_NOQUOTES, "ISO-8859-1");
$font =  'arial.ttf';
imagettftext($im, 20, 0, 10, 20, $black, $font, $text);
imagepng($im);
imagedestroy($im);

结果是这个图像的字母从左到右书写并且不可读，所以如何设置正确的编码以在图像上打印阿拉伯文字