<?php

class ParentClass {

    public function sayHello() {
        echo 'Parent';
    }

}

class ChildrenClassOne extends ParentClass {

    public function sayHello() {
        echo 'Children';
    }

}

class ChildrenClassTwo extends ParentClass {

}


$childrenOne = new ChildrenClassOne;
$childrenOne->sayHello();

$childrenTwo = new ChildrenClassTwo;
$childrenTwo->sayHello();