PHP function that darkens a color

This php function can make the specified color darker

function ColorDarken($color, $dif=20){

 $color = str_replace('#', '', $color);
 if (strlen($color) != 6){ return '000000'; }
 $rgb = '';

 for ($x=0;$x<3;$x++){
 $c = hexdec(substr( $color,(2*$x),2)) - $dif;
 $c = ($c < 0) ? 0 : dechex($c);
 $rgb .= (strlen($c) < 2) ? '0'.$c : $c;
 }

 return '#'.$rgb;
}

//Example:
for ($x=1; $x < 20; $x++) {

 // Start color:
 $c = ColorDarken('#FF481D', ($x * 3));

 print ".n";
}

php