why replace ^m to "\n" or "" removed \t?

Question

I'm using Ubuntu 16.04LTS, HTML, PHP

I make C/C++/java/Python code marking web Site.

I submit this code in HTML

#include <stdio.h>

int main()
{
  int a,b;
  scanf("%d %d",&a,&b);
  printf("%d + %d = %d",a,b,a+b);
  return 0;
}

this text in

<input type="hidden" name="code" id="code">

and use POST method go to PHP

$code = str_ireplace("\x0D", "\n", $_POST['code']);

but server open this code

#include <stdio.h>  int main() { int a,b; scanf("%d %d",&a,&b); printf("%d + %d = %d",a,b,a+b); return 0; }

so I use ubuntu command "sed"

sed -e "s/^M/\n/g" $file > $newfile

but this result

#include <stdio.h>

 int main()
 {
 int a,b;
 scanf("%d %d",&a,&b);
 printf("%d + %d = %d",a,b,a+b);
 return 0;
 }

remove tab and add line first char space

How can I solve this problem?