<?php

/// parent class
interface iA {
    public function execute($str = '');
}

class A implements iA {

    public function __construct() { }

    public function execute($str = '')
    {
        return 'Execution ' . $str;
    }
}

/// child class
interface iB {
    public function execute($str = '');
}

class B extends A implements iB {

    public function __construct()
    {
        parent::__construct();
    }

    public function execute($str = '')
    {
        return parent::execute();
    }

}

var_dump((new B)->execute());