How does XMLReader work in PHP?

Question

PHP

Please explain the features of the Read method.
Let's say there is a code

$xmlUrl = file_get_contents('feed.xml');
  $xmlReader = new XMLReader();
  $xmlReader->xml($xmlUrl);
  while ($xmlReader->read()) {
    if ($xmlReader->name === 'offer') {
      var_dump ($xmlReader->getAttribute('internal-id'));
      $xmlReader->next();
    }
  }
  $xmlReader->close();

Feed example

<offer internal-id="1">
</offer>
<offer internal-id="2">
</offer>
<offer internal-id="3">
</offer>

This code produces the expected result, i.e.
1
2
3

However, if $xmlReader->next() is removed, then the loop will process each offer node twice and I see a duplicate id in the output, i.e.
1
1
2
2
3
3

Could you explain why this behavior occurs?