Check If access_token is valid

I have A Facebook App which auto post birthday reminders.

I stored tokens in database.

But Now most of them are expired.

I am trying to make a PHP file which will find valid tokens from my database and save them.

$query = mysql_query("select fb_token from fb_user")while ($row = mysql_fetch_array($query)) {$url = "https://graph.facebook.com/me?access_token=".$row['fb_token']."";}

By with this Code I'm getting each token. Need Help to Save only valid tokens.