help: input form html, php

hello everyone,

i'd like to have help here with this form of exxercises in this website here

www.englisch-hilfen.de/en/exercises/questions/simple_present.htm

well in this form its seems like a code html but i don't know how it gets results and answers after clicking on checking button or show asnwers and this is the form

 

use print_r($_POST), you can see all the input value after form submit, then validate each input value, if empty, then show sad icon beside that input field

<?php
 
if(isset($_POST['check_solutions'])){
 
if(empty($_POST['solution1_1'])) $error='sad icon';
 
if(empty($_POST['solution2_1'])) $error='sad icon';
 
}
 
?>

Code (markup):

 

yes but the page there is html how did they could put php code into html source?