<?php

Interface IPosts{}
Interface IComments{}

# Models
Class CommentsMysqlModel implements IComments{}
Class CommentsMongoDBModel implements IComments{}

Class PostsMysqlModel implements IPosts{}
Class PostsMongoDBModel implements IPosts{}

# Classes
Class Posts {

    private $postsModel;
    private $commentsModel;

    public function __construct(IPosts $postsModel,IComments $commentsModel){
        $this->postsModel = $PostsModel;
        $this->commentsModel = $commentsModel;
    }
}

// Example Of Usage
# IF I would like to use mysql for both
$comments = new CommentsMysqModel;
$posts = new PostsMysqModel;

$postClass = new Posts($posts,$comments);

// So if i would like to use comments for mongodb

$comments = new CommentsMongoDBModel;
$posts = new PostsMysqModel;

$postClass = new Posts($posts,$comments);


// That's it