First, thank all of you for your guess & tips.
It wouldn't be so weird to find the right answer if I was thinking about the similarity of PHP and C++ syntax.
and one more pleasure, function overriding is enable and working correctly and all the functions are virtual! OO
supportof PHP is great! (except multiple inheritance)
So I 've got the answer, look:
class A {
// you've already known it!
}
class B extends A
{
function f1($x)
{
A::f1($x*4); // ancestor class name double colon as the same in C++
$this->x *= 2;
}
}