file_exists() is useful function can check the availability of the file. Below example enables you to check file availability not only local but also remote file as well.
function file_exists($url)
{
if (preg_match("/http/", $url))
{
$header_response = @get_headers($url, true);
if (preg_match("/200 OK/", $header_response[0])) return true;
else return false;
}
else return (file_exists( $url) ? true:false);
}