PHP mySQL Error

Im new to php so I decided to follow this tutorial for a simple login screen. I got the code setup but when I try login I get this error:

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

The code I got from the tutorial is:

<?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开发者_如何学Go 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();
?>

(Note: All of the mySQL database info is filled in on my version)

Aslo, the author gives a php5 version and a normal php version. I have tried both and gotten the same error. If anyone knows why this is happening help is really appreciated.

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You can print the $sql variable and try the query yourself, or print the mysql error with mysql_error(). This query seems to fail.

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You need to check the return from mysql_query(); you can't assume it worked. If it's FALSE, call mysql_error() to get the error message from MySQL

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this is how i do my DB access in PHP:

$result=mysql_query($sql) or die ("bad sql query: ".mysql_error());

so if the SQL is wrong; i get a usable error, usually with the guilty SQL fragment

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The problem is that mysql_num_rows expects a resource object but if no results are found, it'll just be null. Instead of counting the rows that way, just check if $result === null.