paad/action/action_periodos_select.php

<?php
    $ruta = "../";
    require_once "../include/bd_pdo.php";
    global $pdo;
    $sql = "SELECT * FROM fs_periodo WHERE facultad_id = :idfacultad AND id = :idperiodo";
    $params = [':idfacultad' => $_POST['idfacultad'], ':idperiodo' => $_POST['idperiodo']];
    echo json_encode(query($sql, $params, true));
?>