EmailValueObject.php

<?php

final class EmailValueObject
{
    private $mailbox;
    private $host;

    public function __construct($email)
    {   
        if (false === strpos($email, '@')) {
            throw new \InvalidArgumentException('This does not look like an email');
        }
        list($this->mailbox, $this->host) = explode('@', $email);
    }   

    public function __toString()
    {   
        return sprintf('%s@%s', $this->mailbox, $this->host);
    }

    public function __set($field, $value)
    {
    }

    public function changeMailbox($newMailbox)
    {   
        $copy = clone $this;
        $copy->mailbox = $newMailbox;

        return $copy;
    }   
}

// value object works as expected
$email = new EmailValueObject('john@example.com');
assert((string) $email == 'john@example.com');

// new instance of value object is created as well
$changed = $email->changemailbox('wojtek');
assert((string) $changed == 'wojtek@example.com');
// old is not affected, hasn't been mutated
assert((string) $email   == 'john@example.com');