How to avoid DOM parsing adding html doctype, <head> and <body> tags?

<?
    $string = '
    Some photos<br>
    <span class="naslov_slike">photo_by_ile_IMG_1676-01</span><br />
    <span class="naslov_slike">photo_by_ile_IMG_1699-01</span><br />
    <span class="naslov_slike">photo_by_ile_IMG_1697-01</span><br />
    <span class="naslov_slike">photo_by_ile_IMG_1695-01</span><br />    
    ';

    $dom = new DOMDocument();
    $dom->loadHTML($string);
    $dom->preserveWhiteSpace = false;
    $elements = $dom->getElementsByTagName('span');
    $spans = array();
    foreach($elements as $span) {
        $spans[] = $span;
    }
    foreach($spans as $span) {
        $span->parentNode->removeChild($span);
    }
    echo $dom->saveHTML();


?>

I'm using this code to parse strings. When string is returned by this function, it has some added tags:

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" "http://www.w3.org/TR/REC-html40/loose.dtd">
<html><body><p>Some photos<br><br><br><br><br></p></body></html>

Is there any way to avoid this and to have clean string returned? This input string is just for example, in usage it can be any html string.