extendable singleton class

extendable-singleton.php

<?php

class Singleton
{
    protected static $instanches = [];
    
    private function __construct()
    {
    }
    
    public static function instance()
    {
        if (!isset(self::$instances[static::class])) {
            self::$instanches[static::class] = new static();
        }
        
        return self::$instanches[static::class];
    }
}

class Foo extends Singleton
{
    private $a = 'a';
    
    public function get()
    {
        echo $this->a;
    }
}

class Bar extends Singleton
{
    private $b = 'b';
    
    public function get()
    {
        echo $this->b;
    }
}

echo Foo::instance()->get(); // a
echo Bar::instance()->get(); // b