Object of class .. could not be converted to string

Question

I made my first class and I'm having trouble converting the objects back into strings.

class Cryption
{
    var $data;
    var $salt;

function __construct($data, $salt)
{
    $this->data = $data;
    $this->salt = $salt;
}

function sha512()
{
    $sodium = 'Na';
    return hash_hmac("sha512", $this->data . $this->salt, $sodium);
}

function encrypt()
{
    $salt = substr(sha512(($this->key), 'brownies'), 0, 30);
    return base64_encode(mcrypt_encrypt(MCRYPT_RIJNDAEL_256, $salt, $this->data, MCRYPT_MODE_CBC, md5($salt)));
}

When I use it:

$password = new Cryption(mysql_real_escape_string(trim($_POST['password'])), 'pepper');
$password->sha512();

It says 'PHP Catchable fatal error: Object of class Cryption could not be converted to string'

I don't really know how to get it back into a string. May someone please help me?

Thank you.

Edit:

<?php
require("config.php");
include("includes/cryption/cryption.php");

$username = mysql_real_escape_string(trim($_POST['username']));
$password = new Cryption(mysql_real_escape_string(trim($_POST['password'])), 'pepper'); //use a different salt next time such as a special salt for each user
$password->sha512();

$result = mysql_query("SELECT * FROM `administrators` WHERE username='$username' and password='$password'");
$row = mysql_fetch_row($result);
$count = mysql_num_rows($result);

if ($count == 1) {
    if (isset($_POST['remember'])) {
        session_start();
        $_SESSION['user'] = array(
            'id' => $row[0],
            'username' => $row[1],
            'password' => $row[2]
        );
        $userid = new Cryption($_SESSION['user']['id'], 'kkfishing');
        $session = new Cryption($_SESSION['user']['username'], 'kkfishing');
        $validated = new Cryption($_SESSION['user']['password'], 'kkfishing');

        setcookie("uniqueid", $userid->encrypt(), time() + 60 * 60 * 24 * 100, "/"); //100 days
        setcookie("kksessionid", $session->encrypt(), time() + 60 * 60 * 24 * 100, "/");
        setcookie("kkuserid", $validated->encrypt(), time() + 60 * 60 * 24 * 100, "/");//disguised cookie name
    }
    session_start();
    $_SESSION['authenticated'] = $row[0];
    echo '1'; //true
    exit;
}
else
{
    echo '0'; //false
    exit;
}
?>

Answer 1

The same error can be encountered when initializing the constructor variable in a wrong way like below:

 function __construct($data, $salt)
{
    $this->$data = $data; //mistakenly using $ (dollar) sign after `$this->`
    $this->$salt = $salt;
}

Where the correct way of initializing is:

function __construct($data, $salt)
{
    $this->data = $data;
    $this->salt = $salt;
}

Answer 2

Look at this lines:

$password->sha512();

$result = mysql_query("SELECT * FROM `administrators` WHERE username='$username' and password='$password'");

$password is an object. It should be:

$pw = $password->sha512();

$result = mysql_query("SELECT * FROM `administrators` WHERE username='$username' and password='$pw'");