<?php
/*
Plugin Name: Prueba
Plugin URI: http://wordpress.org/plugins/hello-dolly/
Description: This is not just a plugin, it symbolizes the hope and enthusiasm of an entire generation summed up in two words sung most famously by Louis Armstrong: Hello, Dolly. When activated you will randomly see a lyric from <cite>Hello, Dolly</cite> in the upper right of your admin screen on every page.
Author: Matt Mullenweg
Version: 1.7.2
Author URI: http://ma.tt/
*/

function mostrarFormulario1() {
    //Puedo directamente devolver un string
    return 'Hola Mundo!!!';
}

add_shortcode( 'mostrar_formulario1', 'mostrarFormulario1');

function mostrarFormulario2() {
    //Puedo capturar la salida y usar echos
    ob_start();
    echo "Hola, ";
    ?>
        acá puedo meter html y mesclarlo con <?php echo "php"; ?>
    <?php
    return ob_get_clean();
}

add_shortcode( 'mostrar_formulario2', 'mostrarFormulario2');

function mostrarFormulario3() {
    ob_start();
    //O puedo directamente incluir un archivo php
    include __DIR__ . '/algo.php';
    return ob_get_clean();
}

add_shortcode( 'mostrar_formulario3', 'mostrarFormulario3');