$(document).ready(function(){
$('#form').submit(function(){
$.ajax({
    type: 'POST',
    url: 'ajax/login.php',
    dataType: 'text',
    data: $('#form').serialize(),
    traditional: true,
    success: function(response) {
if (response == 'yes') {
    $("#oKmessage").fadeIn(1000);
    $("#form").hide(1000);
    window.location.reload();
} else if (response == 'no') {
    $("#nomessage").fadeIn(1000);
    $("#form").hide(1000);
} else
    $("#emptymessage").fadeIn(1000);
    $("#form").hide(1000);
},
error: function(response) {
/* ошибка соединения с интернетом */
}
});
return false;
})

});


<?php
require "../core.php";

if (isset($_POST["username"]) && isset($_POST["password"])) {
if ($_POST["username"] != '' or $_POST["password"] != '') {

$username = mysqli_real_escape_string($connect, $_POST["username"]);
$password = hash('sha256', $_POST["password"]);
$check    = mysqli_query($connect, "SELECT username, password FROM
`players` WHERE `username`='$username' AND password='$password'");
if (mysqli_num_rows($check) > 0) {
    session_start();
    $_SESSION['username'] = $username;
    $autch = 'yes';
} else {
    $autch = 'no';
}
echo $auth;
}

}

?>

if (isset($_POST["signin"])) {