相对网址不适用于getheaders

here is this script which works just fine for absolute url in img src tag but if img src tag is having relative url it fails and give error

Warning: get_headers() [function.get-headers]: This function may only be used against URLs in in first link img src is relative url but in second link img src is absolute url and it gives perfect result but i want my script to work in both cases any idea??

<?php
$websitelink = 'http://img172.imagevenue.com/img.php?image=90465_Gwen3_122_17lo.jpg';

//  $websitelink='http://www.santabanta.com/bollywood/76432/whoa-after-200-crore-now-dhoom-3-races-for-rs-300-crore-mark/';

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $websitelink);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
$resulty = curl_exec($ch);
$doc = new DOMDocument();
@$doc->loadHTML($resulty);
$tags = $doc->getElementsByTagName('img');
foreach ($tags as $tag) {
    $data = get_headers($tag->getAttribute('src'), 1);
    if ((intval($data["Content-Length"])) >= 30720) {
        $op7 = '' . $tag->getAttribute('src') . '';
        echo $op7 . '</br>';
    }
}

The function get_headers will actually perform a HTTP GET request, therefore it needs a fully qualified address, and cannot work if you specify only a relative URL.

You will have to infer what's the complete address to the image you're trying to load.