I've notice that with accented characters (so french !!) 
like that:
strlen("câble" * imagefontwidth(FONTSIZE));
this command give a string length bigger than it is in reality
so you have to pass the chain in utf8 decode before

strlen(utf8_decode("câble") * imagefontwidth(FONTSIZE));

that's all (sorry for my english !)

This library function is very useful for variable-sized images that only contain text, like this function that I use to output error messages that accumulate and cause a fatal error in my thumbnailer:

<?php
function errimg($error) {
   // $error is an array of error messages, each taking up one line
   // initialization
   $font_size = 2;
   $text_width = imagefontwidth($font_size);
   $text_height = imagefontheight($font_size);
   $width = 0;
   // the height of the image will be the number of items in $error
   $height = count($error);

   // this gets the length of the longest string, in characters to determine
   // the width of the output image
   for($x = 0; $x < count($error); $x++) {
      if(strlen($error[$x]) > $width) {
         $width = strlen($error[$x]);
      }
   }
   
   // next we turn the height and width into pixel values
   $width = $width * $text_width;
   $height = $height * $text_height;
   
   // create image with dimensions to fit text, plus two extra rows and
   // two extra columns for border
   $im = imagecreatetruecolor($width + ( 2 * $text_width ), 
                              $height + ( 2 * $text_height ) );
   if($im) {
      // image creation success
      $text_color = imagecolorallocate($im, 233, 14, 91);
      // this loop outputs the error message to the image
      for($x = 0; $x < count($error); $x++) {
         // imagestring(image, font, x, y, msg, color);
         imagestring($im, $font_size, $text_width, 
                     $text_height + $x * $text_height, $error[$x], 
                     $text_color);
      }
      // now, render your image using your favorite image* function 
      // (imagejpeg, for instance)
      out($im, array(), $error);
   } else {
      // image creation failed, so just dump the array along with extra error
      $error[] = "Is GD Installed?";
      die(var_dump($error));
   }
}
?>

The function expects an array of error messages to be passed in, and then outputs an image containing the contents of the array.  This is especially useful if your code is contained in an html page that will display rexes if the images do not render correctly.

This function displays the array in image form with index 0 at the top, and the highest index at the bottom.

You have to write out() yourself though, see imagejpeg, imagepng, etc for good ideas on how to write a decent output function.

a function that is faster than ImageFontWidth for certain uses:

<?
function ImageFontWidthByFilename($filename)
{
    $handle = @fopen($font_locations[$i],"r");
    $c_wid = @fread($handle,11);
    @fclose($handle);
    return(ord($c_wid{8})+ord($c_wid{9})+ord($c_wid{10})+ord($c_wid{11}));
}

echo "./font.gdf is ".ImageFontWidthByFilename("./font.gdf")." pixels wide";

?>

reading the widths of 5 different fonts, 500 times, ImageFontWidth took an average of ~0.004 seconds per 5 reads, my function takes ~0.0003 per 5.

The reason is that ImageFontWidth requires a call to ImageLoadFont, but if for some reason you won't need to load the font, just find out the width, this is the function for you :-)

more such wonders at http://puremango.co.uk