javascript - PHP and AJAX login form

Hello
I know this subject has been proposed before, but no one has done the AJAX manipulation standard like i did. I'm newbie to AJAX and jQuery

The problem I have here is that I'm not sure that the js function "ajaxPostCallManip()" reaches to "login.php"!!

I've made a couple of alert()s but nothing appears...Please help

HTML CODE

    
    
    

    


    

        
Login or             data-transition="flip">Register
    
    

        

             
                            

                  
                    
                       Username
                                                     placeholder="username" />

                       Password
                                                     placeholder="password" />

                       


                                            
                                  Submit
                            
                                                            style="display: none; font-size: 12px; 
                                 text-align: center;">status
                       
                
                  
             

        
    

js CODE

    // AJAX Call handler using method="POST"
    function ajaxPostCallManip( str, url, toDoFunc )

    {   
        var xmlhttp;                // Request variable
        if( window.XMLHttpRequest )         // For modern browsers
        xmlhttp = new XMLHttpRequest;
        else                    // For old browsers
        xmlhttp = new ActiveXOBject("Microsoft.XMLHttp");

        xmlhttp.onreadystatechange=toDoFunc;
        xmlhttp.open("POST", url, true);
        xmlhttp.send(str);

    }

    function check_login()
    {
        // Construct the POST variables [username, password]
        var postStr = "username=" + $('#login_username').val() + "&" 
            + "password=" + $('#login_pwd').val();

        // Call the general purpose AJAX Handler Function
        ajaxPostCallManip(postStr, "login.php", function()  

        // toDoFunc to be performed when server response is ready
        {
        if( xmlhttp.readyState == 4 && xmlhttp.status == 200 )
        {   
            alert(xmlhttp.responseText);        
            switch(xmlhttp.responseText)
            {
                case "1":
                $('#login_err').css({'color':'green','display':'block'})
                                       .html('Successful Login');                   

                break;

            case "2":
                    $('#login_err').css({'color':'red','display':'block'})
                                       .html('incorrect username/password')
                break;

            case "3":
                $('#login_err').css({'color':'red','display':'block'})
                                       .html('please fill in all fields')

                break;
            }
        }
        });
    }

PHP CODE

    include('dbManip.php');

    echo '';

    $username = $_POST['username'];
    $password = $_POST['password'];

    // Just to check that the POST variables arrived safely
    echo "";    



    if(!empty($username) && !empty($password))
    {
        // Fetch data from database
        $query = "
            SELECT username,password
            FROM users
            WHERE username = '$username' and password = '$password';
        ";  


        // Execute query
        $res = mysql_query($query);

        // If there is a match for the credentials entered with the database
        if(mysql_num_rows($res) == 1)
        {
            // Fetch information and double check credentials
            while($row = mysql_fetch_assoc($res))
            {
                $db_username = $row['username'];

                $db_password = $row['password'];
            }

            // Compare results with user input
            if( $username == $db_username && $password == $db_password)
            {
                // Credentials are correct - response = 1
                echo '1';
            }
            else 

            {
                // Credentials are incorrect - response = 2
                echo '2';
            }
        }
        else
        {
            // There is no match in the database
            // Credentials are incorrect - response = 2
            echo '2';

        }
    }   
    else 
    {
        // If one or both fields are empty - response = 3
        echo '3';
    }
?>