Para implementar una API Rest es necesario verificar la petición, procesar y devolver los datos en formato JSON para que la APP pueda consumirlas. 

Implementaremos básica con las funciones necesarias para nuestro proyecto.

Login

<?php
$response= array();
if(isset($_POST['login'])){
    require_once "../procesos/dbmanager.php";
    $user = new Dbmanager();
    $_email = $_POST['email'];
    $_password = md5($_POST['password']);
    $data = "email='".$_email."' and password='".$_password."'";
    $u = $user->buscar("users",$data);
    if($u){
        foreach($u as $r){
            $response['id']         =   $r['id'];
            $response['success']    =   true;
        }        
    }else{
        $response['success'] = false;
    }
    $user = null;
}else{
    $response['success'] = false;
}
die(json_encode($response));

Registro

<?php
$response= array();
if(isset($_POST['registro'])){
    require_once "../procesos/dbmanager.php";
    $user       = new Dbmanager();
    
    $_nombre     = $_POST['nombre'];
    $_email     = $_POST['email'];
    $_password  = md5($_POST['password']);
    $_nivel     = 0;

    $data = "'".$_nombre."','".$_email."','".$_password."',".$_nivel;
    $u = $user->insertar("users",$data);
    if($u){
        $response['id'] = $user->lastInsertId();
        $response['success'] = true;
    }else{
        $response['success'] = false;
    }
    $user = null;
}else{
    $response['success'] = false;
}
die(json_encode($response));

Data

<?php
$response= array();
if(isset($_POST['data'])){
    require_once "../procesos/dbmanager.php";
    $user       = new Dbmanager();
    
    $u = $user->buscar("categories","1");
    if($u){
        $response['success'] = true;
        $response['listacategorias'] = array();
        foreach($u as $r){
            $item = array();
            $item['id'] = $r['id'];
            $item['nombre'] = $r['nombre'];
            array_push($response['listacategorias'],$item);
        }
    }else{
        $response['success'] = false;
    }


    $u = $user->buscar("products","1");
    if($u){
        $response['success'] = true;
        $response['listaproductos'] = array();
        foreach($u as $r){
            $item = array();
            $item['id'] = $r['id'];
            $item['nombre'] = $r['nombre'];
            $item['descripcion'] = $r['descripcion'];
            $item['precio'] = $r['precio'];
            $item['stock'] = $r['stock'];
            $item['imagen'] = $r['imagen'];
            $item['categoria_id'] = $r['category_id'];
            array_push($response['listaproductos'],$item);
        }
    }else{
        $response['success'] = false;
    }


    

    $user = null;
}else{
    $response['success'] = false;
}
die(json_encode($response));