The class NotSoImmutableClass demonstrates how-to implement the Immutable Object pattern incorrectly.

NotSoImmutableClassExample.php

<?php
namespace FlorianWolters;

/**
 * The class {@see NotSoImmutableClass} demonstrates how-to implement the
 * *Immutable Object* implementation pattern **incorrectly**
 *
 * @author    Florian Wolters <wolters.fl@gmail.com>
 * @copyright 2013 Florian Wolters
 * @license   http://gnu.org/licenses/lgpl.txt LGPL-3.0+
 * @link      http://gist.github.com/FlorianWolters/5108452
 */
final class NotSoImmutableClass // The class is final and commented. Looks good!
{
    /**
     * The datetime of this object.
     *
     * @var DateTime
     */
    private $dateTime;

    /**
     * Constructs a new object.
     *
     * @param DateTime $dateTime The datetime.
     */
    public function __construct(DateTime $dateTime)
    {
        // TODO Should be:
        // $this->dateTime = clone $dateTime;
        $this->dateTime = $dateTime;
    }

    /**
     * Returns the datetime of this object.
     *
     * @return DateTime The datetime.
     */
    public function getDateTime()
    {
        // TODO Should be:
        // return clone $this->dateTime;
        return $this->dateTime;
    }
}

// Lets check if the class is immutable.

// Lets start by testing the ingoing value of the class...

$constructorArgument = new DateTime('1985-09-28');    // We create the argument for the constructor.
$obj = new NotSoImmutableClass($constructorArgument); // We create a new instance of the class.
echo $obj->getDateTime()->format('Y-m-d'), \PHP_EOL;  // 1985-09-28 - OK!
$constructorArgument->setDate(2001, 9, 11);           // We modify the argument object.
echo $obj->getDateTime()->format('Y-m-d'), \PHP_EOL;  // '2001-09-11'-> First fail!

// Uhh, lets try it with the outgoing value of the class...

$constructorArgument = new DateTime('1985-09-28');
$obj = new NotSoImmutableClass($constructorArgument);
$getterReturnValue = $obj->getDateTime();             // We retrieve the return object via the accessor method.
echo $getterReturnValue->format('Y-m-d'), \PHP_EOL;   // '1985-09-28' -> OK!
$getterReturnValue->setDate(2013, 3, 7);              // We modify the return object.
echo $obj->getDateTime()->format('Y-m-d'), \PHP_EOL;  // '2003-03-07' -> Second fail!

// Uhh again. Looks like that class isn't THAT immutable...