nu exista tagul </input>
HTML input tag

Pt validarea url-ului se poate folosi FILTER_VALIDATE_URL (PHP FILTER_VALIDATE_URL Filter)

do.php
<?
$link = $_POST['link'];
if(!filter_var($link, FILTER_VALIDATE_URL))
{
echo "Link is not valid";
}
else
{
echo "Link is valid";
}
?>