Building a Survey Data Collection project in PHP

I'm building a survey data collection. Currently, I am done with building the prototype. It serves as wizard like control to navigate through various pages in a survey project.

app/index.php

$f3 = require_once __DIR__ . '/lib/f3/base.php';
require_once __DIR__ . '/controllers/PlayerController.php';


function glob_call($f3,$params){
    $player_controller = new PlayerController($f3,$params);
    $player_controller->displayForm();
}

$f3->route('GET|POST /project_id/@project_id',
    'glob_call'
);
$f3->run();

app/controllers/PlayerController.php

require_once __DIR__ . '/../lib/Player.php';


class PlayerController{

    public $player;
    private $f3;

    function __construct($f3,$params){

    $this->f3  = $f3;

    $project_id = $params["project_id"];
    $resp_id = $f3->get("REQUEST.resp_id");



    $player = new Player($project_id,$resp_id);

    $action = $resp_id = $f3->get("POST.navigation");

    if($action==="next"){
        $player->nextPage();  
    }elseif ($action==="prev") {
        $player->prevPage();
    }

    $this->player = $player;

    }


    public function displayForm(){
        $this->f3->set('player',$this->player);
        echo \Template::instance()->render('/templates/template.html');
    }
}

app/lib/Page.php

require_once __DIR__ . '/rb_dbo.php';

    class Page{
        function __construct($page_id,$project_id){

            $this->id = $page_id;
            $this->project_id = $project_id;
            if($this->id===NULL){
                $this->id = PageDBO::get_first($project_id);    
            }
        }

        function getNextPage(){
            $this->id = PageDBO::get_next($this->id,$this->project_id);
            return $this->id;
        }

        function getPrevPage(){
            $this->id = PageDBO::get_prev($this->id,$this->project_id);
            return $this->id;
        }

    }

    class PageDBO{    
        public static function get_first($project_id){
            $first_id = R::getCell("
                        SELECT id from page
                        where project_id = ?
                        order by order_id asc
                        limit 1
                        ",[$project_id]);
            return $first_id;
        }

        public static function get_last($project_id){
            $last_id =  R::getCell("
                        SELECT id from page
                        where project_id = ?
                        order by order_id desc
                        limit 1
                        ",[$project_id]);
            return $last_id;
        }

        public static function get_next($id,$project_id){
            $next_id =  R::getCell("
                        select id from page where order_id  > 
                        (select order_id from page where id = ? and project_id = ? )
                        and project_id = ?
                        order by order_id asc
                        limit 1
                        ",[$id,$project_id,$project_id]);
            if($next_id===NULL){
                return PageDBO::get_last($project_id);
            }
            return $next_id;
        }


        public static function get_prev($id,$project_id){
            $prev_id =  R::getCell("
                        select id from page where order_id  < 
                        (select order_id from page where id = ? and project_id = ? )
                        and project_id = ?
                        order by order_id desc
                        limit 1
                        ",[$id,$project_id,$project_id]);
            if($prev_id===NULL){
                return PageDBO::get_first($project_id);
            }
            return $prev_id;
        }

    }

app/lib/Player.php

require_once __DIR__ . '/Project.php';
require_once __DIR__ . '/Page.php';
require_once __DIR__ . '/Respondent.php';

class Player{


        public $project;
        public $repsondent;
        public $page;

        function __construct($project_id,$repsondent_id){


            $this->project = new Project($project_id);
            $this->repsondent = new Respondent($repsondent_id,$this->project->id);
            $this->page = new Page($this->repsondent->last_page_id,$this->project->id);

            $this->repsondent->last_page_id = $this->page->id;
        }
        function nextPage(){
            $this->repsondent->update_last_page($this->page->getNextPage());
        }

        function prevPage(){
            $this->repsondent->update_last_page($this->page->getPrevPage());
        }

    }

app/lib/Project.php

require_once __DIR__ . '/rb_dbo.php';

    class Project{

        public $id;
        public $title;

        protected function bind(){
                $temp_obj = ProjectDBO::find($this->id);
                $this->id = $temp_obj->id;
            }

        function __construct($project_id){
            $this->id = $project_id;

            $this->bind();

            $this->title = "The First one";
        }
    }

    class ProjectDBO{
        public static function find($id){
                $db_obj = R::findOne('project', ' id = ?', [ $id ] );
                if($db_obj===NULL){
                        throw new Exception($id . ": Not Found");
                }
                return $db_obj;
        }

    }

app/lib/Respondent.php

require_once __DIR__ . '/rb_dbo.php';

class Respondent{

        public $id;
        public $project_id;
        public $last_page_id;

        protected function bind(){
                $resp_temp_obj = RespondentDBO::find($this->id);
                $this->last_page_id = $resp_temp_obj->last_page_id;
        }

       function update_last_page($nav_num){
                $this->last_page_id = RespondentDBO::update_last_page($this->id,$nav_num);
            }

       function __construct($repsondent_id = NULL,$project_id){
                $this->id = $repsondent_id;
                $this->project_id = $project_id;


                if(is_null( $this->id) || empty($this->id)){
                    $this->id = RespondentDBO::create_respondent($this->project_id);
                }

                $this->bind();
            }
        }
class RespondentDBO{
            public static function create_respondent($project_id){
                $db_obj = R::dispense( 'respondent' );
                $db_obj->project_id = $project_id;
                $id = R::store( $db_obj );
                return $id;
            }

            public static function find($id){
                $db_obj = R::findOne('respondent', ' id = ?', [ $id ] );
                if($db_obj===NULL){
                        throw new Exception($id . ": Not Found");
                }
                return $db_obj;
            }

            public static function update_last_page($id,$nav_num){
                $db_obj = R::findOne('respondent', ' id = ?', [ $id ] );
                $db_obj->last_page_id = $nav_num;
                R::store( $db_obj );
                return $nav_num;
            }
    }

Here is the initial DB schema diagram:

I want to make sure that the highly decoupled approach taken is fine for the Player class in "Data Collection/lib/Player.php". Furthermore, if there are ways to better the code and room for improvements.