Example method that cleaned text of HTML and bad characters

cleanText.php

/**
 * Utility function - clean incoming text and strip it of HTML and bad characters.
 * @param $text
 * @return string
 */
public function cleanText($text) {
  if (!drupal_strlen($text)) {
    return '';
  }

  libxml_use_internal_errors(TRUE);
  $document = new DOMDocument();
  $document->loadHTML($text);
  $document->preserveWhiteSpace = FALSE;
  $output = '';
  $xpath = new DOMXPath($document);

  // save the document HTML string
  $document->saveHTML();

  $nodes = $document->getElementsByTagName('body')->item(0);

  foreach ($nodes->childNodes as $node) {
    $node->normalize();

    if (trim($node->textContent, "\xc2 \n \t ") != '' && trim($node->textContent, "\xc2 \n \t ") != ' ') {
      $output .= trim($node->C14N());
    }
  }

  $output = str_replace('<p>&nbsp;</p>', '', $output);
  $output = str_replace('<span>&nbsp;</span>', '', $output);
  $output = str_replace('<span>_</span>', '', $output);
  $output = str_replace('<span> </span>', '', $output);
  $output = str_replace('</img>', '', $output);
  $output = str_replace('<p> </p>', '', $output);
  $output = str_replace(chr(130), "'", $output);    // baseline single quote
  $output = str_replace('”', '"', $output);    // baseline double quote
  $output = str_replace(chr(133), '...', $output);  // ellipsis
  $output = str_replace('‘', "'", $output);    // left single quote
  $output = str_replace('’', "'", $output);    // right single quote
  $output = str_replace('“', '"', $output);    // left double quote
  $output = str_replace('”', '"', $output);    // right double quote
  $output = strip_tags($output);
  $output = preg_replace('/(?!\n)[\p{Cc}]/', ' ', $output);
  $output = preg_replace('/[\x00-\x1F\x7f-\xFF]/', ' ', $output);
  $output = preg_replace('/[^\da-z -.,]/i', '', $output);
  $output = preg_replace(array('/\s{2,}/', '/[\t\n]/'), ' ', $output);
  $output = mb_convert_encoding($output, 'UTF-8', 'UTF-8');

  return $output;
}