ImageCreate中文字符问题

[hf_linux]

<?php
header ("Content-type: image/png");
$im = @imagecreate (50, 100)
or die ("Cannot Initialize new GD image stream");
$background_color = imagecolorallocate ($im, 255, 255, 255);
$text_color = imagecolorallocate ($im, 233, 14, 91);
imagestring ($im, 1, 5, 5, "A Simple Text String", $text_color);
imagepng ($im);
?>
如上面的函数,如果我把"A Simple Text string"换为中文,无法正常显示,请问有什么好办法?