The PHP-GTK Community Site has posted about an issue that's been found with PHP-GTK on a Windows Vista machine that can cause problems with how the OS handles memory (by Wim Stockman).
My work station is WinXp system and my friend where I had to create it for has the new Vista and somewhere over 9000 pictures to be managed. On my system everything worked fine, but on my friends system i always got the error can not open file.
Further testing revealed that it was the OS at fault - Vista wasn't handling the memory usage for the application correctly. When it was run directly from the file explorer (versus in the PHP-GTK console) though, it worked just fine.