<?php

class A {
    private $foo;
    private static $bar = 'bar';
    
    public function __construct() {
        $this->foo = 'foo';
        self::$bar = 'lel';
    }
    
    public function getFoo() {
        return $this->foo;
    }
    
    public static function test1() {
        return $this->foo;
    }
    
    public static function test2() {
        return self::$foo;
    }
    
    public static function test3() {
        return self::$bar;
    }
}

$a = new A();
print_r($a->getFoo());
//print_r($a->test1()); // Uncaught Error: Using $this when not in object context
//print_r($a::test1()); // Uncaught Error: Using $this when not in object context
//print_r($a::test2()); // Uncaught Error: Access to undeclared static property
print_r($a::test3());

<?php

class Singleton {
    private static $instance = null;
    private $foo;
    
    // Watch out for __clone() and such
    private function __construct() {
        // Called exactly one time in getInstance()
        $this->foo = 'foo';
    }
    
    public static function getInstance(): Singleton {
        if (self::$instance === null) {
            self::$instance = new self;
        }
        return self::$instance;
    }
    
    public function getFoo() {
        return $this->foo;
    }
}

//$singleton = new Singleton(); // Uncaught Error: Call to private Singleton::__construct()
print_r(Singleton::getInstance()->getFoo());