you get that error because you are not properly urlencoding the variables you insert into the url. use urlencode() or http_build_query(), eg
$url="https://graph.facebook.com/debug_token?" . http_build_query ( array (
'input_token' => $fb_token,
'access_token' => $access_token
) );
or
$url="https://graph.facebook.com/debug_token?input_token=" . urlencode ( $fb_token ) . "&access_token=" . urlencode ( $access_token );
and it will be properly urlencoded.
solved How Can i pass multiple parameter dynamically get request to check whether fbtoken is valid or not