使用 PHP 的 MySQL 更新不起作用

一切正常，只有 UPDATE 命令不起作用：

$con = mysql_connect($dburl, $dbuser, $dbpass);
if (!$con) {
  die('Could not connect: '.mysql_error());
}
mysql_select_db($dbname, $con);
$result = mysql_query("SELECT * FROM $dbtable WHERE user = '$user'");
$token = generateToken();
if (!empty($result)) {
  // check for empty result
  if (mysql_num_rows($result) > 0) {
    $result = mysql_fetch_array($result);
    if (strcmp($result["pass"], $pass) == 0) {
      $client = array();
      $client["Credential"] = $token;
      mysql_query("UPDATE $table SET credential='$token'
        WHERE user='$user' && pass='$pass'", $con);
      print(json_encode($client));
    }
    else {
      $client["success"] = "0";
      print(json_encode($client));
    }
  }
  else {
    // no product found
    $client["success"] = "0";
    print(json_encode($client));
  }
}
else {
  $client["success"] = "0";
  print(json_encode($client));
}
mysql_close($con);

我需要更新登录名中的凭据令牌。问题与mysql_query（"更新$table设置凭据='$token' 其中用户='$user' && pass='$pass'"， $con）;