PHPで簡単な中国語認証コード機能を実装する方法

<?php
session_start();
/*for($i=0;$i<4;$i++) {
  $rand .= dechex(rand(1,15));
}
$_SESSION[check_pic] = $rand;
*/
$image = imagecreatetruecolor(100, 30);
$bg = imagecolorallocate($image, 0, 0, 0);
$color = imagecolorallocate($image, 255, 255, 255);
//imagestring($image, rand(1,6), rand(3,60), rand(3,15), $rand, $color);
for($i=0;$i<3;$i++) {
  $color2 = imagecolorallocate($image, rand(0,255), rand(0,255),rand(0,255));
  imageline($image, rand(0,100), 0, 100, 30, $color2);
}
//rand() ---->0-max 不大于100
for($i=0;$i<200;$i++) {
  imagesetpixel($image, rand()%100, rand()%30, $color2);
}
//$str = iconv("gbk", "utf-8", "中");
$str = "中国";
$_SESSION[check_pic] = $str;
//解决中文，页面本身为utf-8
$str = mb_convert_encoding($str, "html-entities","utf-8" );
//2:字体大小 3：倾斜角度 x , y 坐标
imagettftext($image, 12, 0, 20, 20, $color, &#39;MSYH.TTF&#39;, $str);
//输出图片
header("Content-type: image/jpeg;charset=utf-8");
imagejpeg($image);
/*修改eclipse的配置，可以使得eclipse的新建项目的默认编码直接为UTF-8
在菜单栏的
Window->Preferences->General->Workspace->Text file encoding
将其改为UFT-8即可。*/
?>

<?php
  header("Content-type: text/html;charset=utf-8");
  session_start();
  if($_POST[check]) {
    if($_POST[check]==$_SESSION[check_pic]) {
      echo "验证码正确：".$_SESSION[check_pic];
    } else {
      echo "验证码错误：".$_SESSION[check_pic];
    }
  }
?>
<form action="" method="post">
  <img alt="" src="img.php"><br/>
  <input type="text" name="check"><br/>
  <input type="submit" value="提交">
</form>