چگونه یک سیستم لوگین برای سایت بسازیم

<?php

//get posted data 
$username = $_POST['username'];
$password = $_POST['password'];

//send query to mysql 
$mysql_command = "select * from 'users table' where username = '$username' and password = '$password'";
$mysql_query   = mysql_query($command);

//fetch the rows number
$mysql_return_rows = mysql_num_rows($mysql_query);

//if rows was more than 0, then the password and username was true.
if ($mysql_return_rows > 0)
{
$_SESSION['username'] = $username;
$_SESSION['password'] = $password;
}else{   //else, the login was wrong.
echo "your username or password is incorrect";
}

?>

if (!$_SESSION['username'] && !$_SESSION['password'])
{
//do not show the private data
}else{
//show the private data
}

<form name="login" method="post" action="login.php">
username: <input type="text" name="username"><p>
password: <input type="password" name="password"><p>
<input type="submit" name="login" value="login">
[B]<input type="hidden" name="command" value="check_access">[/B]
</form>

<?php
$command = $_POST['command'];

//check for page task
if (isset($command))
{
check_user_and_pass();
}else{
show_login_form();
}

?>

<?php 

//get posted data  
$username = $_POST['username']; 
$password = $_POST['password']; 

//send query to mysql  
$mysql_command = "select * from 'users table' where username = '$username' and password = '$password'"; 
$mysql_query   = mysql_query($command); 

//fetch the rows number 
$mysql_return_rows = mysql_num_rows($mysql_query); 

//if rows was more than 0, then the password and username was true. 
if ($mysql_return_rows > 0) 
{ 
$_SESSION['username'] = $username; 
$_SESSION['password'] = $password; 
}else{   //else, the login was wrong. 
echo "your username or password is incorrect"; 
} 

?>