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Put simply, the __construct()
method needs to be limited to scope of the class. Sadly, we cannot guarantee this in
PHP versions prior to 5.
In our example, we use the access doctag, @access, to indicate that the constructor is
private. If we were building this class specifically for a PHP 5 or above environment,
we would be able to use access modifiers (visibility). For more information about
access modifiers, refer to http://php.net/manual/language.oop5.visibility.php.
In the declaration of the getInstance() method we make the method return a
reference and we define it as static in the doctags. This means when we use the
method we must always use the =& assignment operator, to prevent copying of
the returned object, and we must use the method in the static form SomeClass::
getInstance().
At the start of the getInstance() method we declare a new static variable. Unlike
normal variables, static variables do not die after a function or method has completed.
We use the variable as a long-term store to remember the singleton object.


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