This simple code checks that domain name exist. Only enter any URL in variable $URL and run php code. 

<?php
$URL = "http://www.dotnetindex.com/articles/5261-Article--AJAX-Tips-and-Tricks.asp";
$PARSED_URL = parse_url($URL);
$DOMAIN = $PARSED_URL['host'];
$ip = gethostbyname($DOMAIN);
if($ip===$DOMAIN)
{
echo "Url does not exist";
}
else
{
echo "Url exists";
}
?>