Create a waiting page with PHP and twig - php

Question

I post data to a page and make some checks that take 5-6 seconds. I would like to insert a waiting page to improve the user experience.
My code is like this:
....functions that take time
echo $twig->render('template.html.twig',[ variables ....]);
Because PHP calls the twig template at the end after processing the data I cannot use a javascript solution.
I tried rendering a waiting template first, then process the data and store the output in a session variable then after that send a location header to the results page but I found PHP does not echo the waiting template untill it finishes the whole script even if i call it in the beginning.
echo $twig->render('waiting.html.twig',[ variables ....]);
....functions that take time
store output as session variable.
send location header to another page that renders the template from the session variable
How can I achieve a waiting page?

Answer 1

You could always store the data temporarily and load a dummy "loading page" for the user. And right when the dummy page loads you send an ajax request that recovers your data and processes it. When the ajax call returns you could do your redirection or whatever it is you want to do when the process is done.
When I say "store the data temporarily" I mean in a database or a file, etc.

Answer 2

The solution I ended up doing was the following:
Call the page by Ajax and display a waiting page.
function submit_form(file_method) {
var spinner = $('#loader');
spinner.show(); //show waiting div
var request = $.ajax({
url: "upload.php",
cache: false,
contentType: false,
processData: false,
async: true,
data: form_data,
type: 'POST',
success: function (res, status) {
if (status == 'success') {
window.location.href = 'results.php';
} },
error: function (jqXHR, textStatus,res) {
spinner.hide();
alert('Error encountered: '+textStatus+'-'+jqXHR.responseText);
} })
};
In the php page, store the output as an array in a session variable.
....functions that take time
$_SESSION['result'] = [RESULTS .......]
After the ajax call is completed successfully the user is redirected to a new page. The new page uses the session variable to call the template.
echo $twig->render('waiting.html.twig',$_SESSION['result'] );
unset($_SESSION['result']);

Answer 3

Simplest solution is to add 'waiting page' inside first page but hide it. When user presses button browser will send request, but will still wait for response showing old page. Here you can show it using JS.
In short - user presses button, you show template (which was already there but hidden) and then browser just waits for response with your template in front.
But best way would be to use AJAX like Patriot suggested.