关于mysql_query的问题，百思不得其解，望高手回答！

<?php

require_once('./mysql_class.php');

$db = new DataBase();

$db->connect('127.0.0.1', 'root', 'qy7ttvj7vg', 'test');

$nowtime = time();

$subtime = $nowtime-15*60;

$sql = 'delete from online where activetime<'.$subtime;

$db->query($sql);

if(!isset($_COOKIE['username']) || $_COOKIE['username'] == '访客'){

  setcookie('username','访客');

  $log = 'logout';

}

else{

  $log = 'login';

}

$username = $_COOKIE['username'];

$sql = 'select * from online where ipaddr=\''.$_SERVER['REMOTE_ADDR'].'\'';

echo ($query = $db->query($sql))?'YES<br/>':'NO<br/>'; //这里返回值是 TRUE

if($db->num_rows($query)>0){

  $sql = 'update online set username=\''.$username.'\', activetime='.$nowtime.' where ipaddr=\''.$_SERVER['REMOTE_ADDR'].'\'';

}

else{

  $sql = 'insert into online (ipaddr, username, logintime, activetime) values (\''.$_SERVER['REMOTE_ADDR'].'\', \''.$username.'\', '.$nowtime.', '.$nowtime.')';

}

echo $sql;

echo db->query($sql)?'<br/>YES':'<br/>NO'; //问题：这里为什么返回值是FALSE??百思不得其解，望高手回答！

?>

//下面是 数据库类的一部分

class DataBase{

  var $link; //连接号

  //连接数据库 connect('127.0.0.1', 'root', 'pass', 'dbname', 3306)

  function connect($dbhost, $dbuser, $dbpass, $dbname = '', $dbport = 3306){

    if(!$this->link = @mysql_connect($dbhost, $dbuser, $dbpass)){

	  echo 'Can not connect to MYSQL server.';

	  exit();

	}

	if($dbname){

	  @mysql_select_db($dbname, $this->link);

	}

  }

  function query($sql){ //向MYSQL数据库发出$sql语句

    return @mysql_query($sql, $this->link);

  }

}