imagestring() with .gdf file on Solaris 10

We can't get the imagestring() function to work with non-built-in
fonts. It works fine with built-in fonts (1-5). We have tried a
number of .gdf files unsuccessfully. We know the .gdf files are
loading successfully because we can use GD functions to report their width 
and height correctly.

We are running fully patched Solaris 10 and PHP-5.2.6. We have run tests 
both against PHP using the bundled GD library and also against PHP linked 
against a separately built GD 2.0.35 library.

The following script fails:

$font = imageloadfont('anonymous.gdf');
$fontWidth = imagefontwidth($font);
$fontHeight = imagefontheight($font);
$text = 'Hello World';
$im = imagecreate(strlen($text) * $fontWidth, $fontHeight);
$bgColor = imagecolorallocate($im, 255, 255, 255);
$fgColor = imagecolorallocate($im,   0,   0, 255);
imagestring($im, $font, 0, 0, $text, $fgColor);
header('Content-Type: image/png');
imagepng($im);

The above script works if you change the first line to:

$font = 5;

-Leif