01 to 1

function change01($n):
  $arr01 = str_split($n);
  $arr1 = array();
  foreach($arr01 as $num){
    if($num==0){
      $arr1[] = 1;
    }else{
      $arr1[] = 0;
    }
  }
  $r = implode("", $arr1);
  return $r;
  
change01("0100")