PHP mySQL错误

我是PHP的新手,所以我决定遵循this tutorial进行简单的登录.我得到了代码设置,但是当我尝试登录时出现此错误：

Warning: MysqL_num_rows(): supplied argument is not a valid MysqL result resource in (a long file path to the script) on line 27

我从教程中获得的代码是：

<?PHP
ob_start();
$host="thehost"; // Host name
$username="myusername"; // MysqL username 
 $password="mypass"; // MysqL password 
 $db_name="test"; // Database name 
 $tbl_name="members"; // Table name

// Connect to server and select databse.
 MysqL_connect("$host", "$username", "$password")or die("cannot connect"); 
 MysqL_select_db("$db_name")or die("cannot select DB");

// Define $myusername and $mypassword 
 $myusername=$_POST['myusername']; 
 $mypassword=$_POST['mypassword'];

// To protect MysqL injection (more detail about MysqL injection)
 $myusername = stripslashes($myusername);
 $mypassword = stripslashes($mypassword);
 $myusername = MysqL_real_escape_string($myusername);
 $mypassword = MysqL_real_escape_string($mypassword);

 $sql="SELECT * FROM $tbl_name WHERE username='$myusername' and password='$mypassword'";
 $result=MysqL_query($sql);

// MysqL_num_row is counting table row
 $count=MysqL_num_rows($result);
// If result matched $myusername and $mypassword, table row must be 1 row

 if($count==1){
// Register $myusername, $mypassword and redirect to file "login_success.PHP"
 session_register("myusername");
 session_register("mypassword"); 
 header("location:login_success.PHP");
}
 else {
 echo "Wrong Username or Password";
}

 ob_end_flush();
?>

(注意：所有的MysqL数据库信息都填写在我的版本中)

Aslo,作者给出了PHP5版本和普通的PHP版本.我都尝试过并得到相同的错误.如果有人知道为什么会这样,将不胜感激.

解决方法:

这就是我在PHP中进行数据库访问的方式：

$result=MysqL_query($sql) or die ("bad sql query: ".MysqL_error());

因此,如果sql错误；我收到一个有用的错误,通常是有罪的sql片段