Warning: imagettfbbox() [function.imagettfbbox]: Invalid font filename

求助：Warning: imagettfbbox() [function.imagettfbbox]: Invalid font filename
//check we have the appropriate variable data
//variables are button-text and color

$button_text=$_REQUEST['button_text'];
$color=$_REQUEST['color'];

if((empty($button_text)||empty($color))||(!($color=='red'
  ||$color=='blue'||$color=='green')))
{
  echo 'Could not create image-form not filled out correctly';
  exit;
 }
 //create an image of the right background and check size
 $im=imagecreatefrompng($color.'-button.png');
 if(!$im)
 {
  echo 'Could not create image';
  exit;
  }
 $width_image=imagesx($im);
 $height_image=imagesy($im);
 
 //our images need an 18 pixel margin in from the edage of the image
 $width_image_wo_margins=$width_image-(2*18);
 $height_image_wo_margins=$height-(2*18);
 //work our if the font size will fit and make itsmaller until it does
 //start our with the biggest size that will reasonably fit on our buttons
 $font_size=3;
 //you need to tell GD2 where fonts reside
 putenv('GDFONTFATH=C:\windows\fonts');
 $fontname='Arial';
 
 do
 { 
  $font_size--;
  //find out the size of the text at that font size
  $bbox =imagettfbbox($font_size,0,$fontname,$button_text);
  
  $right_text=$bbox[2];//right co-ordinate
  $left_text=$bbox[0];
  $width_text=$right_text-$left_text; //how wide is it?
  $height_text=abs($bbox[7]-$bbox[1]);//how tall is it?
  }
  while($font_size>8&&($height_text>$height_image_wo_margins||
  $width_text>$width_image_wo_margins)
);
 
  if($height_text>$height_image_wo_margins||
  $width_text>$width_image_wo_margins)
  {
  //no readable font size will fit on button
  echo 'Text given will not fit on button.
';
  }
  else
  {
  //we have found a font size that will fit
//now work out where to put it
$text_x=$width_image/2.0-$width_text/2/0;
$text_y=$height_image/2.0-$height_text/2.0;

if($left_text $text_x+=abs($left_text);
$above_line_text=abs($bbox[7]);
$text_y+=$above_line_text;

$text_y-=2;
$white=imagecolorallocate($im,255,255,255);
imagettftext($im,$font_size,0,$text_x,$text_y,$white,$fontname,
$button_text);
header('content-type:image/png');
imagepng($im);
}
imagedestroy($im);
?>
 
 Warning: imagettfbbox() [function.imagettfbbox]: Invalid font filename in E:\AppServ\www\make_button.php on line 38


------解决方案--------------------
没有那个字体 换一个字体或把字体文件拷到相对应的目录
------解决方案--------------------
是字体变量的问题。把 
putenv('GDFONTFATH=C:\windows\fonts');
$fontname='Arial';

直接改成
$fontname = 'c:/windows/fonts/arial.ttf'就可以了，大小写不敏感。