[Logician]

<?php
if (!$url) {echo "No URL";exit;}

$fp = @fopen("$url","r");
if ($fp)
{
echo "The address valid!";
}
else
{
echo "The address invalid";
}
?>

is the code very basically doing the same thing.. Save it to a file (eg.online.php) and upload it to your server. Then call your file like

url/online.php?url=http://www.domain.com/filetocheck.html

it will return "address is valid" message if the url(page) exits.

Of course you can enhance the code if you want, that is just the algorithm in PHP.