How to fix mcrypt character length on PHP Class

I'm here to explain my website and show the "bug" or "error" that appears to my web.

Some days ago I did a page, the topic is encrypt and decrypt with AES, using the php class called "mcrypt".

Here's a screenshot:

Fist Image

The problem is: the page only allow me to put a key length of "16, 24 or 32" characters.

So when I choose another character length it appears an error. Example: If I choose a 8 key character length it appears:

Error or Bug that I want to fix

I was searching on internet and I found that I have to pad with "\0"

I found this

But I wonder, where exacly do this.

Here's the php class:

<?php

class AES {

    const M_CBC = 'cbc';
    const M_CFB = 'cfb';
    const M_ECB = 'ecb';
    const M_NOFB = 'nofb';
    const M_OFB = 'ofb';
    const M_STREAM = 'stream';

    protected $key;
    protected $cipher;
    protected $data;
    protected $mode;
     protected $IV;

    /**
     * 
     * @param type $data
     * @param type $key
     * @param type $blockSize
     * @param type $mode
     */
    function __construct($data = null, $key = null, $blockSize = null, $mode = null) {
        $this->setData($data);
        $this->setKey($key);
        $this->setBlockSize($blockSize);
        $this->setMode($mode);
        $this->setIV("");
    }

    /**
     * 
     * @param type $data
     */
    public function setData($data) {
        $this->data = $data;
    }

    /**
     * 
     * @param type $key
     */
    public function setKey($key) {
        $this->key = $key;
    }

    /**
     * 
     * @param type $blockSize
     */
    public function setBlockSize($blockSize) {
        switch ($blockSize) {
            case 128:
                $this->cipher = MCRYPT_RIJNDAEL_128;
                break;

            case 192:
                $this->cipher = MCRYPT_RIJNDAEL_192;
                break;

            case 256:
                $this->cipher = MCRYPT_RIJNDAEL_256;
                break;
        }
    }

    /**
     * 
     * @param type $mode
     */
    public function setMode($mode) {
        switch ($mode) {
            case AES::M_CBC:
                $this->mode = MCRYPT_MODE_CBC;
                break;
            case AES::M_CFB:
                $this->mode = MCRYPT_MODE_CFB;
                break;
            case AES::M_ECB:
                $this->mode = MCRYPT_MODE_ECB;
                break;
            case AES::M_NOFB:
                $this->mode = MCRYPT_MODE_NOFB;
                break;
            case AES::M_OFB:
                $this->mode = MCRYPT_MODE_OFB;
                break;
            case AES::M_STREAM:
                $this->mode = MCRYPT_MODE_STREAM;
                break;
            default:
                $this->mode = MCRYPT_MODE_ECB;
                break;
        }
    }

    /**
     * 
     * @return boolean
     */
    public function validateParams() {
        if ($this->data != null &&
                $this->key != null &&
                $this->cipher != null) {
            return true;
        } else {
            return FALSE;
        }
    }

     public function setIV($IV) {
        $this->IV = $IV;
    }

     protected function getIV() {
    if ($this->IV == "") {
        $this->IV = mcrypt_create_iv(mcrypt_get_iv_size($this->cipher, $this->mode), MCRYPT_RAND);
    }
        return $this->IV;
    }

    /**
     * @return type
     * @throws Exception
     */
    public function encrypt() {

        if ($this->validateParams()) {
            return trim(base64_encode(
                            mcrypt_encrypt(
                                    $this->cipher, $this->key, $this->data, $this->mode, $this->getIV())));
        } else {
            throw new Exception('Invlid params!');
        }
    }

    /**
     * 
     * @return type
     * @throws Exception
     */
    public function decrypt() {
        if ($this->validateParams()) {
            return trim(mcrypt_decrypt(
                            $this->cipher, $this->key, base64_decode($this->data), $this->mode, $this->getIV()));
        } else {
            throw new Exception('Invlid params!');
        }
    }

}

I also found this, I think could be the solution

function pad_key($key){
    // key is too large
    if(strlen($key) > 32) return false;

    // set sizes
    $sizes = array(16,24,32);

    // loop through sizes and pad key
    foreach($sizes as $s){
        while(strlen($key) < $s) $key = $key."\0";
        if(strlen($k) == $s) break; // finish if the key matches a size
    }

    // return
    return $key;
}

Maybe this last one is the fix, by the way, I'm a little amateur with php.

Thanks to everyone! Hope u can help me.