PHP - recursividad con array's PHP

function listarUrls($url){

	$urlContent = file_get_contents($url);

    $dom = new DOMDocument();

    @$dom->loadHTML($urlContent);

    $xpath = new DOMXPath($dom);

    $hrefs = $xpath->evaluate("/html/body//a");

    $result = '';

    for($i = 0; $i < $hrefs->length; $i++){

        $href = $hrefs->item($i);

        $url = $href->getAttribute('href');

        $url = filter_var($url, FILTER_SANITIZE_URL);

        if(!filter_var($url, FILTER_VALIDATE_URL) === false){

        	$result .= $url."\n";

        }

    }

    return $result;

}

function listarUrls($url, $iteration = 0){

	$urlContent = file_get_contents($url);

    $dom = new DOMDocument();

    @$dom->loadHTML($urlContent);

    $xpath = new DOMXPath($dom);

    $hrefs = $xpath->evaluate("/html/body//a");

    $result = '';

    for($i = 0; $i < $hrefs->length; $i++){

        $href = $hrefs->item($i);

        $url = $href->getAttribute('href');

        $url = filter_var($url, FILTER_SANITIZE_URL);

        if(!filter_var($url, FILTER_VALIDATE_URL) === false){

        	$result .= $url."\n";

        	if ($iteration > 1) {

        		$result .= listarUrls($url, $iteration - 1);

        	}

        }

    }

    return $result;

}