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    <p>Go with a class property. There are a few advantages...</p> <pre><code>class Foo { protected static $instance = null; public static function instance() { if (is_null(self::$instance)) { self::$instance = new Foo(); } return self::$instance; } } </code></pre> <p>First, it's easier to perform automated tests. You can create a mock foo class to "replace" the instance so that other classes which depend on foo will get a copy of the mock instead of the original:</p> <pre><code>class MockFoo extends Foo { public static function initialize() { self::$instance = new MockFoo(); } public static function deinitialize() { self::$instance = null; } } </code></pre> <p>Then, in your test cases (assuming phpunit):</p> <pre><code>protected function setUp() { MockFoo::initialize(); } protected function tearDown() { MockFoo::deinitialize(); } </code></pre> <p>This gets around a common gripe with singletons that they are hard to test.</p> <p>Second, it makes your code more flexible. If you ever want to "replace" the functionality at run time in that class, all you need to do is subclass it and replace <code>self::$instance</code>.</p> <p>Third, it allows you to operate on the instance in other static function. This isn't a huge deal for single instance classes (a true singleton) since you can just call <code>self::instance()</code>. But if you have multiple "named" copies (say for database connections or other resources where you want more than one, but don't want to create a new one if they already exist), it becomes dirty because you then need to keep track of the names:</p> <pre><code>protected static $instances = array(); public static function instance($name) { if (!isset(self::$instances[$name])) { self::$instances[$name] = new Foo($name); } return self::$instances[$name]; } public static function operateOnInstances() { foreach (self::$instances as $name =&gt; $instance) { //Do Something Here } } </code></pre> <p>One other note, I wouldn't make the constructor private. It will make it impossible to extend or test properly. Instead, make it protected so that you can sub-class if needed and still operate on the parent...</p>
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    1. POImplement a PHP singleton: static class properties or static method variables?
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    1. COYou make some good points. Thanks!
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    2. COWhy o why did you make `instance` return by reference. This allows you to externally replace the singleton instance, which you shouldn't be able to. I'd upvote you otherwise.
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    3. CO@Artefacto: good point. Fixed...
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