Is a singleton class linked to the session in PHP? - php

Question

I have a singleton class that I am using as part of a CAPTCHA system to generate the code and image etc.
I have one script included on the html form that loads the class singleton, generates the code and image, and outputs it to the browser. I then have another script to validate and process the form. This loads the class singleton to retrieve the instance that was created previously, and calls the function to validate the code.
The problem I'm having is that when I'm validating the form the code that was generated on the form has changed or is completely absent when I come to validate it!
I haven't started or stored anything in the php session, but a session is created on the page the form is loaded in. Is the instance of the singleton somehow linked to that session? If it's a named session or something?
OR...have I completely misunderstood how singleton classes work? In which case can anyone tell me how I can retrieve the instance of the class that is created on the html form page to use again to validate the code in the form processing script? - And maybe tell me how I should be using singletons!
Many thanks.

Answer 1

Singletons exist for the duration of the request, not the duration of the session.
The idea of a singleton is to provide access to the same object across all included scripts, without having to use any explicit initialisation logic.
So, the first call to $foo = MyObject::singleton() creates a new MyObject, but the second call will simply return that object instead of creating a new one. This is incredibly useful for classes that access external resources such as databases and files.

Answer 2

OR...have I completely misunderstood how singleton classes work?
Partially. Since PHP has no ASP.NET alike application variables, objects in PHP live as long as the request does, unless serialized (for example in a session).
As a solution to your problem: save the captcha code (or the captcha class, which is a bit of overkill imho) in a session variable, like $_SESSION['captcha'].

Answer 3

No, it's not.
You'd have to serialize your singleton object and store it to the session when your code execution ends. When the next page is displayed, you can unserialize the object from your session.
PHP serializes/unserializes objects automatically when you assign them to a session.
This only works correctly under the precondition that your singleton does not use link identifiers to external resources.
Here is an example implementation taken from the comments in PHP docs
class SessionSingleton {
/**
* Returns an instance of the singleton class.
* #return object The singleton instance
*/
public static function _instance()
{
// Start a session if not already started
Session::start();
if ( false == isset( $_SESSION[ self::$_singleton_class ] ) )
{
$class = self::$_singleton_class;
$_SESSION[ self::$_singleton_class ] = new $class;
}
return $_SESSION[ self::$_singleton_class ];
}
/**
* Destroy the singleton object. Deleting the session variable in the
* destructor does not make sense since the destructor is called every
* time the script ends.
*/
public static function _destroy()
{
$_SESSION[ self::$_singleton_class ] = null;
}
/**
* Initialize the singleton object. Use instead of constructor.
*/
public function _initialize( $name )
{
// Something...
}
/**
* Prevent cloning of singleton.
*/
private function __clone()
{
trigger_error( "Cloning a singleton object is not allowed.", E_USER_ERROR );
}
private static $_singleton_class = __CLASS__;
}