static instance in PHP

Why does the following code print "1,1,1," instead of "4,5,6,"?

class MyClass {
  // singleton instance
  private static $instance = 3;

  function __construct() {
 $instance++;
 echo $instance . ",";
  }
}

for($i = 0; $i < 3; 开发者_运维技巧$i++) {
 $obj = new MyClass();
}

$instance is a local variable, not a static class property. Unlike Java you always must access variables, or properties in theire scope

$var; // local variable
$this->var; // object property
self::$var; // class property

I just saw

// singleton instance

The singleton pattern is usually implemented different

class SingletonClass {
    protected $instance = null;
    protected $var = 3;
    protected __construct () {}
    protected __clone() {}
    public static function getInstance () {
        if (is_null(self::$instance)) { self::$instance = new self(); }
        return self::$instance;
    }
    public function doSomething () {
        $this->var++;
        echo $this->var;
    }
}
$a = SingletonClass::getInstance();
$a->doSomething();

The singleton pattern ensures, that you always interact with exactly one instance of a class.

In your constructor, $instance is not yet defined. You must use:

self::$instance++;
echo self::$instance . ",";

to reference the static property of your class.