how to call a function of another class

[eluser]Sumon[/eluser]
here are two simple classes

Code:

class A extends Controller {
public function FA1()
{
   // need to call B class FB1 function. how ?
}
}
class B extends Controller {
public function FB1()
{
   // some statements
}
}

how could i call FB1() from FA1() ?
Thanks in advance.