db.php

<?php

interface DB {
    public function findOne($query, $params);
    public function findAll($query, $params);
}

interface Repositry {
    public function getById(int $id);
}


class UsersRepositry implements Repositry {
    private $db;
    function __construct(DB $db) {
        $this->db = $db;
    }
    public function getById(int $id) {
        return $this->db->findOne(
            [
                'from' => 'users',
                'where' => [
                    'id = ?'
                ]
            ],
            [ $id ]);
    }
}


class MySQL implements DB {
    public $client;
    function __construct() {
        // MySQLへの接続など
    }
    private function toSQL($query) {
        return /** TODO */;
    }
    public function findOne($query, $params) {
        $sql = $this->toSQL($query);
        return $this->client->query($sql)->exec($params)->next();
    }
    public function findAll($query, $params) {
        $sql = $this->toSQL($query);
        return $this->client->query($sql)->exec($params)->all();
    }
}

class MongoDB implements DB {
    public $client;
    function __construct() {
        // MongoDBへの接続など
    }
    private function toMongoQuery($query, $params) {
        return /** TODO */;
    }
    public function findOne($query, $params) {
        $mongoQuery = $this->toMongoQuery($query);
        return $this->client->find_one($mongoQuery, $params);
    }
    public function findAll($query, $params) {
        $mongoQuery = $this->toMongoQuery($query);
        return $this->client->find($mongoQuery, $params);
    }
}


$mysql = new MySQL(/** TODO */);
// $mongo = new MongoDB(/** TODO */);

$users = new UsersRepositry($mysql);
// $users = new UsersRepositry($mongo);

// $users->getById(1);