drupal_strpos vs native strpos inconsistency

drupal_substr("test", 0, 0) returns "t"
substr("test", 0, 0) returns ""

as far as I understand it's a bug as drupal_substr is utf8 safe substr clone.

the problem can cause problems with code like

 while (strpos($node->title, '"') !== FALSE) { 
      $w = (!$w || $w == '»') ? $w = '«' : $w = '»';          
      $pos = strpos($node->title, '"');
      if ($pos !== FALSE) {
        $node->title = drupal_substr($node->title, 0, $pos) . $w . drupal_substr($node->title, $pos + 1);
      }       
    }  

(I got infinite loop as first symbol couldn't be replaced)

simple patch attached