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?