Solved: PHP - Undefined Index Array Problem

I am taking my first course in PHP and I found some logic I am using to build a Tic-Tac-Toe game. However, for some reason the 0th position of the array is being ignored. Here is my code

<?php

$box = array('', '', '', '', '', '', '', '', '');
if (isset($_POST["submitbtn"])) {
  $box[0] = $_POST["box0"];
$box[1] = $_POST["box1"];
  $box[2] = $_POST["box2"];
  $box[3] = $_POST["box3"];
  $box[4] = $_POST["box4"];
  $box[5] = $_POST["box5"];
  $box[6] = $_POST["box6"];
  $box[7] = $_POST["box7"];
$box[8] = $_POST["box8"];
  echo "<pre>";
print_r($box);
}
 ?>
<html>
  <head>
    <title>Tic-Tac-Toe</title>
  </head>
  <body>
    <form name="TicTacToe" method="post" action="test2.php"
          <?php
                 for ($i=0; $i<=8; $i++) {
  printf('<input type ="text" name="box%s" value="%s">', $i, $box[$i]);
  if($i == 2 || $i == 5 || $i == 8){
    print('<br />');
  }
}
          print('<input type="submit" name="submitbtn" value="Submit">');
          ?>
          
    </form>
  </body>
</html>

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Here is a screenshot of he error.
https://gyazo.com/500b6db6b2ff591c9d80b2f21aa03370

As you can see when I put 'a' into the first element of the array it is showing it in the second element. I want to know how it can be corrected to be in the first element.

Please keep in mind I can't use techniques I haven't been taught so please keep it simple.

Thanks