PHP. What is the magic?

Question

PHP

Good afternoon,
I've encountered a small problem today. I managed to solve it, but I didn’t understand one thing:
In short, here is a working piece of code

foreach ($this->allowableTags as $tag) {
      foreach ($doc->getElementsByTagName($tag) as $element) {
        $attributes = $element->attributes;
        $attrs = [];
        for ($i = 0; $i < $attributes->length; ++$i) {
          $attrs[] = $attributes->item($i);
        }
        foreach ($attrs as $attr) {
          $this->clearElement($element, $attr);
        }
      }
    }

And here is the non-working

foreach ($this->allowableTags as $tag) {
      foreach ($doc->getElementsByTagName($tag) as $element) {
        $attributes = $element->attributes;
        for ($i = 0; $i < $attributes->length; ++$i) {
          $attr = $attributes->item($i);
          $this->clearElement($element, $attr);
        }
      }
    }

In fact, there is no difference, but in the first example all attributes are removed, but not in the second. Why is that? And what other solutions are there?

Answer 1

Because when an attribute is removed, the rest are renumbered. Let there be three attributes, their numbers will be 0, 1 and 2.

Шаг 1. 
  $i = 0. 
  Удаляем атрибут 0.
  Атрибуты 1 и 2 получают номера 0 и 1.
Шаг 2.
  $i = 1.
  Удаляем атрибут 1.
  Остался атрибут 0.

Well, PHP is not at all to blame here.

foreach ($this->allowableTags as $tag) {
  foreach ($doc->getElementsByTagName($tag) as $element) {
    $attributes = $element->attributes;
    while ($attributes->length > 0) {
      $attr = $attributes->item(0);
      $this->clearElement($element, $attr);
    }
  }
}