Remove selected elements using PHP DOM

<?php
$url = 'http://en.wikipedia.org/w/index.php?title=Elephant&action=render';
$curl = curl_init();
curl_setopt($curl, CURLOPT_URL, $url);
curl_setopt($curl, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($curl, CURLOPT_USERAGENT, 'Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.13) Gecko/20080311 Firefox/2.0.0.13');

// Now, we have the response
$html = '<html>' . curl_exec($curl) . '</html>';
curl_close($curl);

$document = new DOMDocument('1.0');
$document->loadHTML($html);

// these are the elements which are to be kept in the response, rest all are to be removed.
$allowed_elements = array('a','b','i','p');

// this array stores all nodes present in the DOM initially.
$elems = array();

$parent = $document->getElementsByTagName('html')->item(0);
foreach ($parent->getElementsByTagName('*') as $element)
{
    $node = (string)$element->nodeName;
    if(strtolower($node) == 'body'){
        continue;
    }

    // store this node in the array.
    $elems[] = $node;
}

// keep only unique node names, i.e remove multiple occurrences of same node
$elems = array_values( array_unique( $elems ) );

// now, get a list of the elements, which are to be removed from the response
// array_diff removes all the elements present in the second array from the first array
// and returns a final array. So $elems now has all the elements, which need to be removed.
$elems = array_diff( $elems, $allowed_elements );

// re-index and sort them
$elems = array_values( array_unique( $elems ) );
sort($elems);

// Now, remove all elements present in $elem as we don't need them.
foreach( $elems as $elem ) {
    $parent1 = $parent->getElementsByTagName($elem);

    // this is the number of times this element occurs in the response
    $length = $parent->getElementsByTagName($elem)->length;

    // remove each one of them
    for($i=0;$i<$length;$i++) {
        // 0 will always be the index because after each `removeChild`, the next element shifts 1 position back.
        $el = $parent1->item(0);
        if( $el ) {
            $el->parentNode->removeChild($el);
        }
    }
}

echo $document->saveHTML();
?>