Description:
------------
Singleton designed class in PHP 5.1.4 does not really return a single object after instantiated several times. 

My PC computer
PHP 5.1.4
Windows XP SP2
MySQL 5.0.20

Testing Server:  

http://pcdinh.dotgeek.org/singleton/info.php

Reproduce code:
---------------
I have created a singleton class like this:
[php]
class Singleton {

	private static $instance = null;

	private function __construct() {
	}

	public static function getInstance() {
		if (!isset (self :: $instance)) {
			$className = __CLASS__;
			self :: $instance = new $className;
		}
		return self :: $instance;
	}

}
[/php]

The test file 1 is as follows:

[php]
<?php
require_once './Singleton.php';

$singleton = Singleton :: getInstance();
$singleton2 = Singleton :: getInstance();

print "$singleton<br />";
print "$singleton2<br />";
?>
[/php]


The test file 2 is as follows:
[php]
<?php
require_once "Singleton.php";

// This line make problems
$e = new RuntimeException();

$singleton = Singleton :: getInstance();
$singleton2 = Singleton :: getInstance();

print "$singleton<br />";
print "$singleton2<br />";
?>
[/php]







Expected result:
----------------
I expected that test file 1 returns

Object id #1
Object id #1

and test file 2 returns

Object id #1
Object id #1

because the class is designed to return the same object every time. A class based on the Singleton pattern properly instantiates and initializes one instance of the
class and provides access to the exact same object every time. However, it is not the case here.

Actual result:
--------------
Running the test file 1 returns:

Object id #1
Object id #1

That result is Ok. It implies that singleton designed class has worked correctly.

Running the test file 2 right after that returns:

Object id #2
Object id #2

It means that in a same session, same computer and very short interval time of script execution between 2 test file, I have got 2 different results:

Object id #2
Object id #2

and 

Object id #1
Object id #1

So what is the different between 2 test files: I added one line to test file 2: $e = new RuntimeException(); or $e = new Exception(). If I remove them, I will get my expected results.

You can test it here:

http://pcdinh.dotgeek.org/singleton/

I think there is a bug in Zend Engine 2. I get confused with how singleton classes work in a stateless environment as in PHP.