PHP Variable Scope Static properties and variables

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Static class properties that are defined with the public visibility are functionally the same as global variables. They can be accessed from anywhere the class is defined.

class SomeClass {
    public static int $counter = 0;

// The static $counter variable can be read/written from anywhere
// and doesn't require an instantiation of the class
SomeClass::$counter += 1;

Functions can also define static variables inside their own scope. These static variables persist through multiple function calls, unlike regular variables defined in a function scope. This can be a very easy and simple way to implement the Singleton design pattern:

class Singleton {
    public static function getInstance() {
        // Static variable $instance is not deleted when the function ends
        static $instance;

        // Second call to this function will not get into the if-statement,
        // Because an instance of Singleton is now stored in the $instance
        // variable and is persisted through multiple calls
        if (!$instance) {
            // First call to this function will reach this line,
            // because the $instance has only been declared, not initialized
            $instance = new Singleton();

        return $instance;


$instance1 = Singleton::getInstance();
$instance2 = Singleton::getInstance();

// Comparing objects with the '===' operator checks whether they are
// the same instance. Will print 'true', because the static $instance
// variable in the getInstance() method is persisted through multiple calls
var_dump($instance1 === $instance2);

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