PHP如何迭代mysqli查询结果？

Question

I'm learning to interact with a database in PHP. The code below works, but I do not understand how the loop iterates through $result.

<?php
  require_once('./login.php');
  $conn = new mysqli($hn, $un, $pw, $db);
  if ($conn->connect_error) die("DIED: CONNECTION FAILED.");

  $query= "SELECT * FROM table1";
  $result = $conn->query($query);
  if (!$result) die("DIED: QUERY FAILED.");

  while ($row = $result->fetch_array(MYSQLI_ASSOC)) {
    $who = $row['WHO'];
    $what = $row['WHAT'];

    echo <<<_END
    WHO: $who<br>
    WHAT: $what<br>
_END;
  }

  $result->close();
  $conn->close();
?>

I understand that while a $result element is available, it will be assigned to $row and processed in the loop. But how does the interpreter know to move to the next $result element?

The code even works when using a for loop instead, such as...

$row_count = $result->num_rows;
for($j = 0; $j < $row_count; $j++) {
  $row = $result -> fetch_array(MYSQLI_ASSOC);

My question boils down to this: how does the interpreter know to use the next element in $result without needing something like $result[$j]?