function strip_selected_tags($text, $tags = array()) {
  $args = func_get_args();
  $text = array_shift($args);
  $tags = func_num_args() > 2 ? array_diff($args, array($text)) : (array) $tags;
  foreach($tags as $tag) {
    if (preg_match_all('/<'.$tag.
        '[^>]*>([^<]*)</'.$tag.
        '>/iu', $text, $found)) {
      $text = str_replace($found[0], $found[1], $text);
    }
  }

  return preg_replace('/(<('.join('|', $tags).
    ')( | |.)*/>)/iu', '', $text);
}

$str = "[url="] 123[/url]";
    echo strip_selected_tags($str, array('b'));