Singleton pattern example in PHP

Question

I'm a PHP newbie. Here is a standart Singleton pattern example according to phptherightway.com:

<?php class Singleton { public static function getInstance() { static $instance = null; if (null === $instance) { $instance = new static(); } return $instance; } protected function __construct() { } private function __clone() { } private function __wakeup() { } } class SingletonChild extends Singleton { } $obj = Singleton::getInstance(); var_dump($obj === Singleton::getInstance()); // bool(true) $anotherObj = SingletonChild::getInstance(); var_dump($anotherObj === Singleton::getInstance()); // bool(false) var_dump($anotherObj === SingletonChild::getInstance()); // bool(true)

The question is in this line:

 static $instance = null; if (null === $instance) { $instance = new static(); }

So as i understand if (null === $instance) is always TRUE, because everytime i'm calling the method getInstance() the variable $instance is always being set to null and a new instance will be always created. So this is not really a singleton. Could you please explain me ?

static $instance = null; will be executed only on first function call - at others it will be ignored — user3423805
– user3423805, Commented Oct 30, 2014 at 7:40

martindilling · Accepted Answer · 2014-10-30 07:41:44Z

Look at "Example #5 Example use of static variables" here: http://php.net/manual/en/language.variables.scope.php#language.variables.scope.static

"Now, $a is initialized only in first call of function and every time the test() function is called it will print the value of $a and increment it."

user3423805 · Accepted Answer · 2014-10-30 07:41:23Z

See http://php.net/manual/en/language.variables.scope.php#language.variables.scope.static

static $instance = null;

will be runned only on first function invoke

Now, $a is initialized only in first call of function

and all other time it will store created object

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