<?php
    require "../db.php";

    $data = file_get_contents("php://input");
    $requestBody = json_decode($data, true);
    $login = $requestBody['login'];
    $password = $requestBody['password'];

    $user = R::findOne('users', 'login = ?', array($login));
    if($user)
    {
        if(password_verify($password, $user->password))
        {
            $d = array('access'=>true, 'comment'=>'Auhorization successful!');
        }
        else
        {
            $d = array('access'=>false, 'comment'=>'Incorrect password!');
        }
    }
    else
    {
        $d = array('access'=>false, 'comment'=>'User '.$login.' not found!');
    }
    echo json_encode($d);
?>

{"access":false,"comment":"User  not found!"}

import com.hand.tubes.model.AuthorizationResponse;

import retrofit2.Call;
import retrofit2.http.POST;
import retrofit2.http.Query;
import retrofit2.http.Headers;

public interface ApiService {
    @Headers({
            "Content-Type: application/json",
            "Content-Length: 80"
    })
    @POST("auth")
    Call<AuthorizationResponse> authorize(
            @Query("login") String login,
            @Query("password") String password);
}

POST http://ru.stackoverflow.com/ HTTP/1.1
Content-Type: application/x-www-form-urlencoded
cache-control: no-cache
Postman-Token: 0db86e7c-1649-41d4-98b9-19aebdbcf18e
User-Agent: PostmanRuntime/7.3.0
Accept: */*
Host: ru.stackoverflow.com
cookie: prov=e712f6ff-bef2-3787-431c-03d858b78086
accept-encoding: gzip, deflate
content-length: 10
Connection: keep-alive

test=value