From 58b40059d63445dd7564e813774dfa5d7ce7d490 Mon Sep 17 00:00:00 2001
From: supergrecko <business@supergrecko.com>
Date: Sat, 27 Jul 2019 18:31:41 +0200
Subject: [PATCH] Added singleton article

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+---
+authors:
+  - "@supergrecko"
+created_at: "2019/07/27"
+---
+
+# Singletons
+
+A singleton is a class which is only instantiated once during runtime. This is done by keeping a static property containing its instance on the singleton class.
+
+There are multiple benefits to using a singleton class
+
+- You're always going to pull the same instance of the class
+- It's only instantiated once
+- Use `$this` in a static-like context
+- 
+
+By using a singleton instead of a static class we expose a cleaner class to use and we can use regular instance properties instead of static properties.
+
+# Creating a Singleton in PHP
+
+Creating a class which can be used as a singleton is very simple.
+
+```php
+<?php
+
+namespace Example;
+
+class Singleton
+{
+    
+    /**
+     * The singleton instance
+     * @var Singleton
+     */
+    private static $instance;
+    
+    /**
+     * Get the instantiated singleton, or create it if it hasn't been instantiated yet.
+     * @return Singleton
+     */
+    public static function getInstance(): Singleton {
+        // In PHP 7.4 we will be able to do
+        // static::$instance =?? new static();
+        // The double ?'s is a null-coalesce operator. There's a link about it below.
+        static::$instance = static::$instance ?? new static();
+        
+        return static::$instance;
+    }
+
+}
+```
+
+# Testing our Singleton
+
+To give a little functionality to our freshly baked Singleton we add these three members to the class
+
+```php
+    private $word = "Pineapple";
+    
+    public function getWord(): string {
+        // PHP_EOL is a constant for a new line (\r\n) or whichever your OS uses.
+        return $this->word . PHP_EOL;
+    }
+    
+    public function setWord(string $word): void {
+        $this->word = $word;
+    }
+```
+
+We are now ready to test our Singleton.
+
+We'll start of by proving that only one instance is created during runtime. We'll grab the singleton instance twice using `Singleton::getInstance();` and comparing their object ids using `spl_object_hash`. Let's try it!
+
+```php
+$first = Singleton::getInstance();
+$second = Singleton::getInstance();
+
+// Compare the hash ids for each of the variables, if they are equal then they contain the same instance.
+var_dump(spl_object_hash($first) === spl_object_hash($second)); // bool(true)
+```
+
+We can now prove its static-like functionality by using our `getWord` and `setWord` methods.
+
+We will do this by comparing the returned value from `getWord()` on `$first` and `$second`.
+
+```php
+var_dump($first->getWord() === $second->getWord()); // bool(true)
+
+// now let's try chaning the value on $first
+$first->setWord("Banana");
+
+// the test will still pass, because it's a singleton.
+var_dump($first->getWord() === $second->getWord()); // bool(true)
+```
+
+# Further Research
+
+Here's a couple links which will help you understand Singletons better.
+
+## Singleton Resources:
+
+- https://en.wikipedia.org/wiki/Singleton_pattern
+- https://phpenthusiast.com/blog/the-singleton-design-pattern-in-php
+- https://phptherightway.com/pages/Design-Patterns.html#singleton
+
+## Static Keyword Resources:
+
+- https://www.php.net/manual/en/language.oop5.static.php
+- https://www.php.net/manual/en/language.oop5.late-static-bindings.php
+- https://en.wikipedia.org/wiki/Static_(keyword)
+
+## The Null-Coalescing Operator Resources:
+
+- https://en.wikipedia.org/wiki/Null_coalescing_operator
+- https://www.php.net/manual/en/migration70.new-features.php#migration70.new-features.null-coalesce-op