PHP memory references

Question

I am wondering this question for a long time, how does PHP handle references are they a good idea to use and I can't explain better than using an example, lets look at the following class and then @ the comment of the setResult method.

Lets imagine we are using a model view controller framework and we are building a basic AjaxController, we only got 1 action method (getUsers) so far. Read the comments, and I hope my question is clear, how does PHP handle these kind of situations and is it true what I wrote about the x times in the memory @ the setResult docblock.

class AjaxController{
    private $json = array(
        'result' => array(),
        'errors' => array(),
        'debug' => array()
    );

    /**
     * Adds an error, always displayed to users if any errors.
     * 
     * @param type $description 
     */
    private function addError($description){
        $this->json['errors'][] = $description;
    }

    /**
     * Adds an debug message, these are displayed only with DEBUG_MODE.
     * 
     * @param type $description 
     */
    private function addDebug($description){
        $this->json['debug'][] = $description;
    }

    /**
     * QUESTION: How does this go in memory? Cause if I use no references,
     * the array would be 3 times in the memory, if the array is big (5000+)
     * its pretty much a waste of resources.
     * 
     * 1st time in memory @ model result.
     * 2th time in memory @ setResult ($resultSet variable)
     * 3th time in memory @ $this->json
     *
     * @param array $resultSet 
     */
    private function setResult($resultSet){
        $this->json['result'] = $resultSet;
    }

    /**
     * Gets all the users
     */
    public function _getUsers(){
        $users = new Users();
        $this->setResult($users->getUsers());
    }

    public function __construct(){
        if(!DEBUG_MODE && count($this->json['debug']) > 0){
            unset($this->json['debug']);
        }

        if(count($this->json['errors']) > 0){
            unset($this->json['errors']);
        }

        echo json_encode($this->json);
    }
}

Another simple example: What would be better to use technique A:

function example(){
    $latestRequest = $_SESSION['abc']['test']['abc'];

    if($latestRequest === null){
        $_SESSION['abc']['test']['abc'] = 'test';
    }
}

Or technique B:

function example(){
    $latestRequest =& $_SESSION['abc']['test']['abc'];

    if($latestRequest === null){
        $latestRequest = 'test';
    }
}

Thanks for reading and advise :)

Answer 1

In short: don't use references.

PHP copies on write. Consider:

$foo = "a large string";
$bar = $foo; // no copy
$zed = $foo; // no copy
$bar .= 'test'; // $foo is duplicated at this point.
                // $zed and $foo still point to the same string

You should only use references when you need the functionality that they provide. i.e., You need to modify the original array or scalar via a reference to it.