Implementing multiple interfaces in PHP5 and instanceof operator

//interfaces
interface Intf
{

public function f();

}
interface Intg
{

public function g();
//public function f(); // error in case of multiple implementing (with Intf)

}

//simple implementation
class Cf implements Intf
{

public function f(){ print "interface Cf, function f"; }

}
//multiple implementation
class Cfg implements Intf, Intg
{

public function f(){ print "interface Cfg, function f"; }
public function g(){ print "interface Cfg, function g"; }

}

//tipe hinting with interface name
function callF(Intf $obj)// tpye hinting not compulsory
{

$obj->f();
//$obj->g(); // runtime error

}

//pass a class that implements the interface "Intf"
callF(new Cf()); // OUTPUT: interface Cf, function f

//callF("hello");// error, argum. must implement interface Intf

//instanceof sample
function callAll($obj)// type hinting "Intf" or "Intg" don't generate warnings
{

if ($obj instanceof Intf) $obj->f(); // (1) //interface Cfg, function f
if ($obj instanceof Intg) $obj->g(); // (2) //interface Cfg, function g

}
//a "Cfg" object is an instance of both interfaces
callAll(new Cfg());