How to convert a color to its inverse color in php, php converts a color to its inverse color_PHP tutorial

function color_inverse($color){
  $color = str_replace('#', '', $color);
  if (strlen($color) != 6){ return '000000'; }
  $rgb = '';
  for ($x=0;$x<3;$x++){
    $c = 255 - hexdec(substr($color,(2*$x),2));
    $c = ($c < 0) &#63; 0 : dechex($c);
    $rgb .= (strlen($c) < 2) &#63; '0'.$c : $c;
  }
  return '#'.$rgb;
}
//使用范例：
// black -> white
print color_inverse('#000000'); 
// --> returns #ffffff
// blue -> yellow
print color_inverse('#0000FF');
// --> #FFFF00