PHP code to convert a color to its inverse color

This php code can change a color into the opposite color encoding, such as: white becomes black, blue becomes yellow

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) ? 0 : dechex($c);
 $rgb . = (strlen($c) < 2) ? '0'.$c : $c;
 }
 return '#'.$rgb;
}

//Usage example:
// black -> white
print color_inverse('#000000');
// --> returns #ffffff

// blue -> yellow
print color_inverse('#0000FF');
// --> #FFFF00

Convert to, and vice versa, PHP