Why can't I use mysql_num_rows in a function?

[N9ne]

function isAdmin($username, $password)
{
 global $connect, $select_db, $adminstable;
  $query = mysql_query("SELECT userid FROM $adminstable WHERE username = '$username' AND password = '$password'");
  if(mysql_num_rows($query) > 0)
    return true;
  else
    return false;
} 

I get an error thrown at me because of the mysql_num_rows, I'm stumped, I don't know why it's got a problem with it? It works in a normal PHP file but not in a function...

Also, if it's a known problem and rule that I can't have in there, what alternative do I have?

[bonk]

$connect and $select_db are only variables, you still have to make the connection. You can do this outside or inside the function, as long as it's before your use the $query.

[N9ne]

The connection is made before the function...

[bonk]

Oh yeah, sometimes it throws an error if the query is faulty. Print out the query with whatever is being used of username and pass to make sure username, pass and the table variables are set.