A interface class extends another interface class also.

interface A
{
    public function a();
}

interface B extends A
{
    public function b();
}

class C implements A, B
{
    public function a()
    {
        echo "hi A";
    }
    public function b()
    {
        echo "hi B";
    }
}

$c = new C();
$c->a();
$c->b();