PHP - RGB to Y'CbCr/Y'PbPr

/**
  * @param  integer $r       : The red rgb value; must satisfy 0<=$r<256
  * @param  integer $g       : The green rgb value; must satisfy 0<=$g<256
  * @param  integer $b       : The blue rgb value; must satisfy 0<=$b<256
  * @param  boolean $digital : Whether or not the image should conform to
  *                            ITU-R BT.709 standard (Y'CbCr) for digital
  *                            and HDTV display.  Otherwise, ITU-R BT.601/
  *                            CCIR 601 (Y'PbPr) is used
  * @return array            : An array, where "y", "cb", and "cr" contain
  *                            their respective values.
  */
 function rgb2ycbcr($r,$g,$b,$digital=true) {
    $Kb=$digital?0.0722:0.114;
    $Kr=$digital?0.2126:0.299;
    $Y=$Kr*$r+(1-$Kr-$Kb)*$g+$Kb*$b;
    return array(
       "y" =>round($Y),
       "cb"=>round(($b-$Y)/(2*(1-$Kb))),
       "cr"=>round(($r-$Y)/(2*(1-$Kr)))
    );
 }