I don't know how to make it clearer than this. Looks like you may need to do the trick they explain there to make PHP see the font file.
Code:
fontfile
The path to the TrueType font you wish to use.
Depending on which version of the GD library PHP is using, when fontfile does not begin with a leading / then .ttf will be appended to the filename and the library will attempt to search for that filename along a library-defined font path.
When using versions of the GD library lower than 2.0.18, a space character, rather than a semicolon, was used as the 'path separator' for different font files. Unintentional use of this feature will result in the warning message: Warning: Could not find/open font. For these affected versions, the only solution is moving the font to a path which does not contain spaces.
In many cases where a font resides in the same directory as the script using it the following trick will alleviate any include problems.
<?php
// Set the enviroment variable for GD
putenv('GDFONTPATH=' . realpath('.'));
// Name the font to be used (note the lack of the .ttf extension)
$font = 'SomeFont';
?>