I was having trouble trying to render non antialiased text using a pixel font. The tips about setting a negative value for the color are valid, but I was still having trouble with the text I was trying to render because it was black. I discovered that if I changed the imagecolorallocate() function from:
$color = imagecolorallocate($base, 0, 0, 0);
to
$color = imagecolorallocate($base, 1, 1, 1); (near black)
and then used the negative value for the color in imagettftext(), it would work properly. The difference is that my first implementation set $color = 0. Obviously, you can't have $color = -0, it made no difference. When I switched to (1,1,1) it became $color = 1 which I could take a negative value for.