PHP - Extracting variable data from a form

Question 1

I have written a possible solution for your problem - see code below. You can find a working copy of this at http://www.floris.us/SO/codeForm.php

The key to solving your specific problem is right near the bottom; I use

<script type="text/javascript">
document.forms['codeForm'].codeMsg.value = <?php echo '"'.$newString.'"'; ?>;
document.forms['codeForm'].msg.value = <?php echo '"'.$string.'"'; ?>;
</script>

to take the value of a php variable and put it in an input area of the form.

<html>
<form class="form-inline" name="codeForm" role="form" method="post">
    <div class="table-responsive">
        <table>
            <tr class="tr_top">
                <!-- <th class="th_left_top">Message:</th> -->
                <td class="td_top">
                    Plain text message<br>
                    <textarea class="form-control" rows="3" name="msg"
                      <?php 
                       if (isset($_POST['encode'])) {
                         echo 'value = "'.htmlspecialchars($_POST['msg']).'"';
                       } 
                       else {
                         echo 'placeholder = "Your message here"';
                       } 
                       ?>
                       onfocus='this.select()'>
                      </textarea>
                </td>
                <td class="td_top">
                    Coded message<br>
                    <textarea class="form-control" rows="3" name="codeMsg" 
                    <?php 
                    if (isset($_POST['decode'])) {
                      echo "value=".htmlspecialchars($_POST['codeMsg']);
                    } 
                    else {
                      echo 'placeholder = "Encoded message here. "';
                    } 
                    ?> 
                    onfocus='this.select()'>
                    </textarea>
                  </td>
            </tr>
            <tr class="tr_mid">
                <td class="td_mid">
                    <input type=text class="form-control input_mid" name="offset" 
                    <?php 
                    if (isset($_POST['encode'])) { 
                      echo 'value = "'.htmlspecialchars($_POST['offset']).'"';
                    } 
                    else { 
                      echo 'placeholder = "Enter a number:"';
                    } 
                    ?>
                    >
                </td>
            </tr>
            <tr class="tr_bottom">
                <!-- <th class="th_bottom empty"></th> -->
                <td class="td_bottom">
                    <input class="input_bottom btn btn-default" type="submit" name="encode" value="Encode">
                    <input class="input_bottom btn btn-default" type="submit" name="decode" value="Decode">
                    <input class="input_bottom btn btn-default" type=button value='Clear' onclick='this.form.elements.msg.value=""' </td>
            </tr>
        </table>
    </div>
    <!-- close table-responsive -->
</form>

<?php
  $string = defaultKey($_POST, 'msg', '');
  $offset = defaultKey($_POST, 'offset', 0);
  $newString = defaultKey($_POST, 'codeMsg', '');
  if(isset($_POST['encode'])) {
    $newString = caesar($string, $offset);
  }
  if(isset($_POST['decode'])) {
    $string = caesar($newString, -$offset);
  }


function caesar($input, $offset) {
  $output = array();
  foreach(str_split($input) as $k=>$i) {
    if (ctype_alpha($i)) {
      $oldChar = strtoupper($i);
      $newChar = chr(((ord($oldChar) - ord('A')) + $offset) % 26 + ord('A'));
      if($i!=strtoupper($i)) { $newChar = strtolower($newChar); }
    }
    else {
      $newChar = $i;
    } // don't do anything with punctuation
    $output[$k]=$newChar;
  }  
  $output = implode($output);
  return $output;
}

function defaultKey($arr, $key, $default) {
  if(isset($arr[$key])) {
    return $arr[$key];
  }
  return $default;
}

?>
<script type="text/javascript">
document.forms['codeForm'].codeMsg.value = <?php echo '"'.$newString.'"'; ?>;
document.forms['codeForm'].msg.value = <?php echo '"'.$string.'"'; ?>;
</script>       
</html>

Question 2

Your code works; the only problem is that the encoded string may contain non-printing characters which will cause trouble. I just tested it with the following inputs:

enter image description here

The problem is that you really need to make sure that the "encoded" string doesn't contain any non-printing characters. It seems that you are implementing a Caesar cypher - to do so efficiently I would use modulo arithmetic (you used a modulo operation on the offset in your code, then commented it out). Thus

$oldChar = strtoupper($input);
$newChar = (($oldChar - ord('A')) + $offset) % 26 + ord('A');

If you want, you can add a line

if($input!=strtoupper($input)) $newChar = strtolower($newChar);

(as far as I know, php doesn't have a built in "isupper()" function...)

Note - this destroys all punctuation etc. If you don't start by testing that you have a valid character you could introduce all kinds of strange characters (in the example below, the space became a comma...). As it is, if your initial code had ascii value 123 in it, you would be toast.