I found this problem, when I was trying to benchmark my code. It seems that BC Math extension, which is built-in to PHP since version 4.? does not work properly with PHP GTK.
I tried this example both with PHP-GTK and regular PHP and I get different results. I am not sure if this is a missing feature or a bug. My installation is PHP-GTK 2.0.1 from gtk.php.net (binary and extensions).
bcsub (or any other BC Math) function exists, it just doesn't work. I even tried supplying floats directly as arguments - always returns 0.000000.