Possible Duplicate:
How to validate domain name in PHP?
Better way to validate a URL in PHP
How do I check, if $variable is a site address?
Like, for this it should give true:
$varialbe = 'http://google.com';
For this false:
$variable = 'this value can be anything, but we know its not a domain';
Use the filter_var function (also see types of filters) provided by PHP:
$is_url = filter_var($url, FILTER_VALIDATE_URL);