determine which string is longer and then what is different in that string

["Richard Kurth" <richardkurth@xxxxxxxxxxxxxx>]

I am trying to find out which string is the longest and then find out what
is different between the two string. Sometimes String 2 is longer than
String 1 The script below works sometimes but not all the time.
Is there a better way to do this or tell me what is wrong with the script I
am using
 
$string1 = "1,2,3,4,5,6,7,8";
$string2 = "1,2,3,4,6,7,8"; 
 
 
 
function compare_by_length ($a, $b)
{
    $la = strlen ($a);
    $lb = strlen ($b);
    if ($la == $lb) return (0);
    return ($a > $b) ? 1 : 2;
}
 
$string1 = "1,2,3,4,5,6,7,8";
$string2 = "1,2,3,4,6,7,8"; 
 
$longstring=compare_by_length ($string1, $string2);
 
if ($longstring=="0"){
  echo "nochange" . $longstring;
  echo"<br>";
}
if ($longstring=="1"){
  echo "List " . $longstring;
  echo"<br>";
  $array1 = explode(",", $string1);
  $array2 = explode(",", $string2);
}
if ($longstring=="2"){
  echo "List " . $longstring;
  echo"<br>";
  $array1 = explode(",", $string2);
  $array2 = explode(",", $string1);
 
}
 
   for ($i = 0; $i < count($array1); $i++) {
      $associative_array[$array1[$i]] = 1;
   }
   
   // print "Not a member of the following lists:<br>\n";
   for ($i = 0; $i < count($array2); $i++) {
      if (!$associative_array[$array2[$i]]) {
         print "$array2[$i]<br>\n";
      }
   }