Locating tags in a string in PHP (with respect to the string with tags removed)

I want to create a function that labels the location of certain HTML tags (e.g., italics tags) in a string with respect to the locations of characters in a tagless version of the string.
(I intend to use this label data to train a neural network for tag recovery from data that has had the tags stripped out.)
The magic function I want to create is label_italics() in the below code.

$string = 'Disney movies: <i>Aladdin</i>, <i>Beauty and the Beast</i>.';
$string_all_tags_stripped_but_italics = strip_tags($string, '<i>'); // same as $string in this example
$string_all_tags_stripped = strip_tags($string); // 'Disney movies: Aladdin, Beauty and the Beast.'
$featr_string = $string_all_tags_stripped.' '; // Add a single space at the end
$label_string = label_italics($string_all_tags_stripped_but_italics);
echo $featr_string; // 'Disney movies: Aladdin, Beauty and the Beast. '
echo $label_string; // '0000000000000001000000101000000000000000000010'

If a character is supposed to have an <i> or </i> tag immediately preceding it, it is labeled with a 1 in $label_string; otherwise, it is labeled with a 0 in $label_string. (I’m thinking I don’t need to worry about the difference between <i> and </i> because the recoverer will simply alternate between <i> and </i> so as to maintain well-formed markup, but I’m open to reasons as to why I’m wrong about this.)

I’m just not sure what the best way to create label_italics() is.

I wrote this function that seems to work in most cases, but it also seems a little clunky and I’m posting here in hopes that there is a better way. (If this turns out to be the best way, the below function would be easily generalizable to any HTML tag passed in as a second argument to the function, which could be renamed label_tag().)

function label_italics($stripped) {
  while ((stripos($stripped, '<i>') || stripos($stripped, '</i>')) !== FALSE) {
    $position = stripos($stripped, '<i>');
    if (is_numeric($position)) {
      for ($c = 0; $c < $position; $c++) {
        $output .= '0';
      }
      $output .= '1';
    }
    $stripped = substr($stripped, $position + 4, NULL);
    $position = stripos($stripped, '</i>');
    if (is_numeric($position)) {
      for ($c = 0; $c < $position; $c++) {
        $output .= '0';
      }
      $output .= '1';
    }
    $stripped = substr($stripped, $position + 5, NULL);
  }
  for ($c = 0; $c <= strlen($stripped); $c++) {
    $output .= '0';
  }
  return $output;
}

The function produces bad output if the tags are surplus or the markup is badly formed in the input. For example, for the following input:

$string = 'Disney movies: <i><i>Aladdin</i>, <i>Beauty and the Beast</i>.';

The following misaligned output is given.

Disney movies: Aladdin, Beauty and the Beast.
0000000000000001000000000101000000000000000000010

(I’m also open to reasons why I’m going about the creation of the label data all wrong.)