inherited method call

Hello,

my question is related to just PHP not postgres, but I hope you'll be so kind to help me.
So, how can I call an inherited method which is overriden in the derived class?
a short exmple - if my explanation is not so clear to unterstand:

class A {
    var $x;
    function A() { $this->x = 0; }
    function f1($x) { $this->x = ++$x; }
}

class B extends A
{
    function B() {$this->x = 1; }

    function f1($x)
    {
# now I want to access the original f1() function, ie .:
        // $this->f1($x*4);
# and perform some  additional operations ie.:
        // $this->x *= 2;
    }
}

Is there any possilbe work-around
    or I have to duplicate the code of the inherited function,
    or leave functions the same and create new functions which calls the inherited ones?