How can I handle the warning of file_get_contents() function in PHP?

I wrote a PHP code like this

$site="http://www.google.com";
$content = file_get_content($site);
echo $content;

But when I remove “http://” from $site I get the following warning:

Warning:
file_get_contents(www.google.com)
[function.file-get-contents]: failed
to open stream:

I tried try and catch but it didn’t work.