RGB & Hex colors

[Hoyo]

I start with a base color and need to get two other colors based on that one: a 50% lighter and a 50% darker.
From the hex value of the base color, I run the following for each R, G, B channel:

$hexcolor="#FFCC00"; //the base color

$rgb["r"]=substr($hexcolor, 1, 2); //hex value for red channel
$rgb["g"]=substr($hexcolor, 3, 2); //hex value for green channel
$rgb["b"]=substr($hexcolor, 5, 2); //hex value for blue channel

$light="#";
$dark="#";

foreach($rgb as $channel=>$hex_value)
{
   $hex_value=hexdec($hex_value); //convert hex to dec to do maths
   $light.=dechex((255-$hex_value)/2+$hex_value);
   $dark.=dechex($hex_value/2);
}

This works OK, but some colors(especially the darker ones) spit out undesired results: dark blue=>red (with certain values).

Are there more accurate maths for acheiving this?

[JDcrack]

You should run a check to make sure r/g/b is not greater than 255 or less than 0.

  	  	if ($rgb["r"]>255) $rgb["r"]=255;
	if ($rgb[g]>255) $rgb[g]=255;
	if ($rgb[b]>255) $rgb[b]=255;
	if ($rgb[r]<0) $rgb[r]=0;
	if ($rgb[g]<0) $rgb[g]=0;
	if ($rgb[b]<0) $rgb[b]=0;