Converts an RGB triplet to a hex color.
function _color_pack($rgb, $normalize = FALSE) {
$out = 0;
foreach ($rgb as $k => $v) {
$out |= $v * ($normalize ? 255 : 1) << 16 - $k * 8;
}
return '#' . str_pad(dechex($out), 6, 0, STR_PAD_LEFT);
}