It should return FALSE.
See here: file_get_contents
'On failure, file_get_contents() will return FALSE.'
You can always test it by entering a fake URL, i.e a URL that cannot be reached.
You can check the response code (to see if it's a 404) by using:
file_get_contents("http://example.com");
var_dump($http_response_header);
This is taken from the answer to this question: HTTP requests with file_get_contents, getting the response code
http://php.net/manual/en/reserved.variables.httpresponseheader.php