You could try this (if you want to search the whole page content) :
$digipart ="http://www.digipart.com/part/".$searchkeyword;
$pageContent = getHtml($digipart);
if (strpos($pageContent, $string) !== false) {
echo 'Not Available.';
} else {
echo 'Available';
}
function getHtml($url, $post = null) {
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, 1);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);
if(!empty($post)) {
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_POSTFIELDS, $post);
}
$result = curl_exec($ch);
curl_close($ch);
return $result;
}