PHP Multiple Inheritance with Interfaces

I'm trying to understand how using interfaces gives me multiple inheritance as I've been googling.

class A
{
 function do1(){}
 function do2(){}
 function do3(){}
}

class B extends A
{
 function do4(){}
 function do5(){}
 function do6(){}
}

class C extends B
{
}

In the above example, class C has all the methods from class A and B. However, class B also has all the methods of class A, which is not necessary desired.

My searches have come up to use interfaces to solve this issue by moving methods to a class and creating interfaces, as below.

interface A
{
     function do1();
     function do2();
     function do3();
}

interface B
{
     function do4();
     function do5();
     function do6();
}

class C implements A, B
{
     function do1(){}
     function do2(){}
     function do3(){}
     function do4(){}
     function do5(){}
     function do6(){}
}

I don't really see how this solves the issue because all the code is in the new class. If I just wanted to use class A as originally, I would have to create a new class that implement interface A and copy the same code to the new class.

Is there something I'm missing?