It seems very simple. but very strong to use.
Actually, I used to use it onto my project frequently.

Thanks to_ https://github.com/paulgb/simplediff

function diff($old, $new){

  $matrix = array();
  $maxlen = 0;

  foreach($old as $oindex => $ovalue){

    $nkeys = array_keys($new, $ovalue);
    foreach($nkeys as $nindex){

      $matrix[$oindex][$nindex] = isset($matrix[$oindex – 1][$nindex – 1]) ?
      $matrix[$oindex – 1][$nindex – 1] + 1 : 1;

      if($matrix[$oindex][$nindex] > $maxlen){
        $maxlen = $matrix[$oindex][$nindex];
        $omax = $oindex + 1 – $maxlen;
        $nmax = $nindex + 1 – $maxlen;
      }

    }

  }

  if($maxlen == 0) return array(array(‘d’=>$old, ‘i’=>$new));
  
  return array_merge(
    diff(array_slice($old, 0, $omax), array_slice($new, 0, $nmax)),
    array_slice($new, $nmax, $maxlen),
    diff(array_slice($old, $omax + $maxlen), array_slice($new, $nmax + $maxlen)));

}

function htmlDiff($old, $new, $mode=0){
  $ret = ”;
  $diff = diff(preg_split(“/[s]+/”, $old), preg_split(“/[s]+/”, $new));
  
  foreach($diff as $k){

    if(is_array($k))
      if($mode == 0){ //추가/삭제 모두 표시
        $ret .= (!empty($k[‘d’])?”<span class=’deleted’>”.implode(‘ ‘,$k[‘d’]).”</span> “:”).
        (!empty($k[‘i’])?”<span class=’inserted’>”.implode(‘ ‘,$k[‘i’]).”</span> “:”);
      }else if($mode == 1){ //삭제 내용만 표시
        $ret .= (!empty($k[‘d’])?”<span class=’deleted’>”.implode(‘ ‘,$k[‘d’]).”</span> “:”);
      }else if($mode == 2){ //추가 내용만 표시
        $ret .= (!empty($k[‘i’])?”<span class=’inserted’>”.implode(‘ ‘,$k[‘i’]).”</span> “:”);
      }else $ret .= $k . ‘ ‘;
    else $ret .= $k . ‘ ‘;

  }

  return $ret;
}