oop - PHP Multiple Inheritance with Interfaces

Question

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?

Answer 1

PHP doesn't have multiple inheritance. If you have PHP 5.4, though, you can use traits to at least avoid every class having to copy code.

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

trait Alike {
    public function do1() { }
    public function do2() { }
    public function do3() { }
}


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

trait Blike {
    public function do4() { }
    public function do5() { }
    public function do6() { }
}


class C implements A, B {
    use Alike, Blike;
}

class D implements A {
    use Alike;

    // You can even "override" methods defined in a trait
    public function do2() { }
}

Note, though, you have to both implement the interface and use the trait (or, of course, provide your own implementation). And C and D are not related at all, except in both implementing the A interface. Traits are basically just interpreter-level copy and paste, and do not affect inheritance.