Caesar Cipher Example (with txt file process)

<?php
/********************************
-- Caesar Chiper Example
-- CREATED BY : CAHYA DSN
-- CREATED ON : 2015-11-17
*********************************/
?>
<!doctype html>
<html>
  <head>
    <title>Caesar Chiper</title>
  </head>
  <body>
<fieldset>
  <legend>Source Data</legend>
  <form enctype="multipart/form-data" method="POST">
  Rotate<input type="number" name='rot' min="1" max="26" />[1-26]<br />
  Send this file: <input name="userfile" type="file" />
  <input type="submit" value="Send File" />
</form>
</fieldset>
<?php
function caesar($str, $n) {
    $ret = "";
    for($i = 0, $l = strlen($str); $i < $l; ++$i) {
        $c = ord($str[$i]);
        if (97 <= $c && $c < 123) {
            $ret.= chr(($c + $n + 7) % 26 + 97);
        } else if(65 <= $c && $c < 91) {
            $ret.= chr(($c + $n + 13) % 26 + 65);
        } else {
            $ret.= $str[$i];
        }
    }
    return $ret;
}

function crack_caesar($str) {
    $max = 0;
    $weight = array(
        6.51, 1.89, 3.06, 5.08, 17.4,
        1.66, 3.01, 4.76, 7.55, 0.27,
        1.21, 3.44, 2.53, 9.78, 2.51,
        0.29, 0.02, 7.00, 7.27, 6.15,
        4.35, 0.67, 1.89, 0.03, 0.04, 1.13);
    $c = $s = array(
        0, 0, 0, 0, 0,
        0, 0, 0, 0, 0,
        0, 0, 0, 0, 0,
        0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0);
    for($i = 0, $l = strlen($str); $i < $l; ++$i) {
        $x = (ord($str[$i]) | 32) - 97;
        if (0 <= $x && $x < 26) {
            ++$c[$x];
        }
    }
    for ($off = 0; $off < 26; ++$off) {
        for ($i = 0; $i < 26; ++$i) {
            if ($max < ($s[$off]+= 0.01 * $c[$i] * $weight[($i + $off) % 26])) {
                $max = $s[$off];
            }
        }
    }
    return ((26 - array_search($max, $s)) % 26)+1;
}

if(isset($_FILES['userfile']['name']) && isset($_POST['rot'])){
  $uploadfile=basename($_FILES['userfile']['name']);
  if (move_uploaded_file($_FILES['userfile']['tmp_name'], $uploadfile)) {
    echo "File is valid, and was successfully uploaded.\n";
  } else {
      echo "Possible file upload attack!\n";
  }
  $handle=@fopen($uploadfile,'r');
  $contents = fread($handle, filesize($uploadfile));
  fclose($handle);
  $n=$_POST['rot'];
  $msg=caesar($contents,$n);
  $save_path = 'result'.date('YmdHis').'.txt';
  $fp = @fopen($save_path, 'a');
  fputs($fp, "$msg\n");
  fclose($fp);
?>
<fieldset>
  <legend>Source Text Data</legend>
  <textarea><?php echo $contents;?></textarea>
</fieldset>
<fieldset>
  <legend>Result Text Data</legend>
  <textarea><?php echo $msg; ?></textarea><br />
  Rotate by
  <input type="text" value="<?php echo crack_caesar($msg);?>" />
</fieldset>
<?php
}
?>
</body>
</html>