Login using JQuery, PHP and MySQL data validation issue

I send my login information using ajax/jquery to php where I want to search for the user in the database. If I specify the string in the php file manually it works. It seems like there is an issue with the serialize and json_decode which i am not sure. Can anyone help me ?

I am putting in all the snippets of my html file and php file..

</div><script type="text/javascript">
    $(document).on('pageinit', '#login', function(){ 
    $(document).on('click', '#submit', function() { // catch the form's submit event
        if($('#username').val().length > 0 && $('#password').val().length > 0){
            // Send data to server through the ajax call
            // action is functionality we want to call and outputJSON is our data
                $.ajax({url: 'check.php',
                    data: {action : 'login', formData : $('#check-user').serialize()},
                    type: 'post',                  
                    async: 'true',
                    dataType: 'json',
                    beforeSend: function() {
                        // This callback function will trigger before data is sent
                        $.mobile.showPageLoadingMsg(true); // This will show ajax spinner
                    },
                    complete: function() {
                        // This callback function will trigger on data sent/received complete
                        $.mobile.hidePageLoadingMsg(); // This will hide ajax spinner
                    },
                    success: function (result) {
                        if(result.status) {

                            $.mobile.changePage("#second"); 
                                alert(result.message);
                        } else {
                            alert('Logon unsuccessful!');
                        }
                    },
                    error: function (request,error) {
                        // This callback function will trigger on unsuccessful action               
                        alert('Network error has occurred please try again!');
                    }
                });                  
        } else {
            alert('Please fill all necessary fields');
        }          
        return false; // cancel original event to prevent form submitting
    });   

});

Now the next is my PHP script...

<?php   
// We don't need action for this tutorial, but in a complex code you need a way to determine Ajax action nature
 $action = $_POST['action'];
// Decode JSON object into readable PHP object
 $formData = json_decode($_POST['formData']);

// Get username
$username = $formData->{'username'};
// Get password
$password = $formData->{'password'};

$db = @mysql_connect('.............', '......', '.......') or die("Could not connect database");
@mysql_select_db('.......', $db) or die("Could not select database");


$result = mysql_query("SELECT `password` FROM `userdb` WHERE `username`= '$username'");
$r = mysql_fetch_assoc($result);
$pass_ret = $r['password'];

// Lets say everything is in order
if($action == 'login' && $password == $pass_ret){
$output = array('status' => true, 'message' => 'Login');
}
else
{
$output = array('status' => false, 'message' => 'No Login');

}
echo json_encode($output);

?>