PHP Example from Prof Stone

Here is the example code from Prof Stone, who guest-lectured February 26 and 28. I’ve made some slight modifications to the code.
form_ex.php

<!DOCTYPE html>
<html>
    <head>
        <title>Form Example</title>
    </head>
    <body>
    <form method="post" action="form_ex.php">
      <h2>Day</h2>
        <?php
        $current_day = 31;
        while ($current_day>0) {
            echo "<input type='radio' name='day' value='$current_day'>$current_day ";
            $current_day--;
        }
       ?>
        <h2>Month</h2>
        <?php
        echo "<select name='month'>";
        for ($index=1;$index<13;$index++) {
            echo "<option value='$index'>$index</option>";
        }
        echo "</select>";
         ?>
        <h2>Year</h2>
        <?php
        echo "<select name='year'>";
        for ($index=1950;$index<2025;$index++) {
            echo "<option value='$index'>$index</option>";
        }
        echo "</select>";
         ?>
      <input type="submit" name="submit" value="Search" /><br />
  </form>
  <?php
//    err_reporting(E_ALL);
//    ini_set('display_errors',1);
   $year_value = $_POST['year'];
   if( isset($year_value)) {
    echo "your year value exists! it is $year_value";
   } else {
    echo "no year value &nbsp;";
   }
   echo "<br />";
   $month_value = $_POST['month'];
   if (isset ($month_value)) {
    echo "your month value exists! it is $month_value";
   } else {
    echo "no month value set";
   }
   echo "<br />";
   $day_value = $_POST['day'];
   if (isset ($day_value)) {
    echo "your day value exists! it is $day_value";
   } else {
    echo "no day value set";
   }
   echo "<br />";
   // THIS IS NOT THE SOLUTION, JUST TO DEBUG
   echo $year_value+$month_value+$day_value;
   ?>
    </body>
</html>