paad/action/action_carreras.php

<?php
header('Content-Type: application/json');

$ruta = "../";
require_once "../class/c_login.php";

// check if the session is started
if (!isset($_SESSION['user']))
    die(json_encode(['error' => 'No se ha iniciado sesión']));

$user = unserialize($_SESSION['user']);

$ruta = "../";
require_once "../include/bd_pdo.php";

$nivel = $db->where("id", $_POST['periodo'])->getOne("fs_periodo", "nivel_id");
$carreras = $db
    ->where("nivel", $nivel)
    ->where("facultad", $_POST['facultad'])
    ->get("fs_carrera", null, "id, carrera");

$user->print_to_log("Crea carrera", old: $_POST);

die(json_encode($carreras));