What is the output of the following code?

What is the output of the following code?

class A {

public $a = 1;

public function __construct($a) { $this->a = $a; }

public function mul() {

return function($x) {

return $this->a*$x;

};

}

}

$a = new A(2);

$a->mul = function($x) {

return $x*$x;

};

$m = $a->mul();

echo $m(3);