Alternated Caesar Cipher

I've recently picked up PHP, for the first assignment in my IT-security class I decided to code it in PHP.
The algorithm is an alternation of the Caesar cipher, instead of using a fixed number to push the letters a keyword is applied to rearrange a couple of letters and then move the rest of the letters accordingly.

I am pretty sure there can be improvements in string usage and probably also in the algorithm?
Any ideas and notes on programming style and such are very welcome.

Code:
https://github.com/TheLML/encrypt/blob/master/encrypt.php

or:

```
letters[$i][0] = chr(65+$i);
$this->letters[$i][1] = 0;
}

for($i=26;$iletters[$i][0] = chr(97+$i-26);
$this->letters[$i][1] = 0;
}

for($i=52;$iletters[$i][0] = $i-52;
$this->letters[$i][1] = 0;
}

$this->letters[62][0] = ' ';
$this->letters[62][1] = 0;
}

/**
* Encrypts the key using an alternation of the caesar cipher.
* The key is used to set a different alternation for the letters, instead
* of having it moved by a constant number.
*
* @throws Exception - if there is no key
* @return string - returns the encrypted alphabet as array
*/
private function _encrypt()
{
if(empty($this->key))
{
throw new Exception("Empty key: cannot create encryption without a key.");
}

$key = $this->key;
$letters = $this->letters;

$encryption = array_fill(0, 63, '');
$pos = 0;

//add key to encoding. each letter only once, though
for($i=0;$iletters[$i][1] = 0;
}
return $encryption;
}

/**
* Encrypts the given word.
*
* @throws Exception - if there is no unencrypted word
* @return string - Encrypted word
*/
public function encrypt()
{
if(empty($this->uword))
{
throw

Give the class a better name... a noun makes more sense than a verb.

Consider simply removing the $uword and $eword properties and applicable setter methods. Instead, your encrypt method signature could be:

/**
 * @param string $uword word to encrypt
 * @return string encrypted word
 */
public function encrypt($uword)
...

Do the same with decrypt:

/**
 * @param string $eword word to decrypt
 * @return string decrypted word
 */
public function decrypt($eword)
...

Put a $key argument in the constructor:

public function __construct($key)
{
    $this->key  = $key;
    ...

With the $key invariant satisfied, no need to throw an exception if not set. Remove the setKey/getKey methods (unless you really need them).

Cleaner calling code example:

$cipher = new CeaserCipher('my key, get this from where ever');
$encryptedValue = $cipher->encrypt('foo');

$decryptedValue = $cipher->decrypt($encryptedValue);

StackExchange Code Review Q#24529, answer score: 5