PHP Caesar密码

Question

(jsfiddle example here: caesar cipher)

I am making a Caesar cipher - it's an alphabetic shift, basically; a shift parameter of 3 is as follows: a => d, b => e, c => f, ...

My code so far is

$string = "hello";
echo "<p>" . $string . "</p>";
$newstring = "hello";

for ($i=0;$i<strlen($string);$i++) {

    $ascii = ord($string[$i]);
    if($ascii == 90) { //uppercase bound
        $ascii = 65; //reset back to 'A' 
        } 
    else if($ascii == 122) { //lowercase bound
        $ascii = 97; //reset back to 'a' 
        } 
    else {
        $ascii++;
    }
    $newstring[$i] = chr($ascii);

}

echo "<p>" . $newstring . "</p>";

and it works okay.

I would like a user to be able to enter their own string into a form's textarea as follows:

<form>
    <table>
        <tr><th class="w">Message:</th>
            <td><textarea class="t" onfocus='this.select()' name=msg>HELLO</textarea></td>
        </tr>
        <tr><th>Shift Parameter:</th>
            <td><input type=text size=2 name=sp value='11'></td>
        </tr>
        <tr><td></td><td><input type=submit name=submit value='Encode'>
        <input type=submit name=submit value='Decode'>
        <input type=button value='Clear' onclick='this.form.elements.msg.value=""'</td></tr>
    </table>
</form>

I just need to change the PHP to output the table, but don't quite know how. So, in textarea, a user could type "TESTING", choose a shift parameter of 8, and click "Encode" and that would run an encode function; likewise, "Decode" would run a decode function.

Any ideas how to change the PHP code to make it work the way I'd like?

Answer 1

First,change

$string = "hello";

to

$string = $_POST['msg'];

and

$newstring = "hello";

to

$newstring = $string;

Then, read the value of shift parameter:

$sp = $_POST['sp']

Now, instead of doing

$ascii++;

increment its value by the value of shift parameter. if the value of $sp is 8, the final $ascii value would be $ascii + 8

Or you can do what "elclanrs" has suggested : $char = 'a'; ++$char; //=> "b". But instead of ++$char; do $char+=$sp;

While incrementing the value of $ascii, you must check for the bounds. May be use a loop and increment the value of $ascii by 1.

So Inside your for loop:

for ($i=0;$i<strlen($string);$i++) {...}

there will be another for loop and all the conditions for the bound would go inside it. Something like :

for ($i=0;$i<strlen($string);$i++) {

  $ascii = ord($string[$i]);
  for($j=0;$j<$sp;$j++){
    if($ascii == 90) { //uppercase bound
      $ascii = 65; //reset back to 'A' 
    } 
    else if($ascii == 122) { //lowercase bound
      $ascii = 97; //reset back to 'a' 
    } 
    else {
      $ascii++;
    }
  }
  $newstring[$i] = chr($ascii);

}

(I'm not a PHP developer, so the syntax might be wrong.)