look at the following code :

<?
  $s = chr(0x0a);
  print strlen($s);
  $fp =fopen("test.txt","w");
  fwrite($fp,$s,1);
  fclose($fp);
?>

the output on screen is 1(strlen($s)) but in my file, i get always 0x0d 0x0a.
With Unix, i get only 0x0a in my file.