<div style="height:100%;width:50%;" class='left'>
<form action="" method="POST">
 <input class='username' type='text' name='username'><br>
 <input  class='pw' type='password' name='password'><br>
 <input  class='submit' type='submit' id='login_btn' name='submit'>
</form>
</div>
<div style="height:100%;width:50%;" class='right'>
    </div>

html, body {
  margin: 0;
 padding: 0;
   overflow:hidden;

  }
.left {


background: url(ui/left.png);
position:absolute;
background-position: center left;
background-size:cover;

}

.right {

background: url(ui/right.png);
background-position: center right;
position:absolute;
background-size:cover;
right:0px;


}

input {

outline:none;

  }

  .username {

border:none;
width:565px;
height:69px;
background: url(ui/textbox.png);
margin-top:300px;
margin-left:120px;
font-size:2em;
text-align:center;
color:white;

  }
    .pw{

border:none;
width:565px;
height:69px;
background: url(ui/textbox.png);
margin-top:60px;
margin-left:120px;
font-size:2em;
text-align:center;

  }

      .submit{

border:none;
width:446px;
height:85px;
background: url(ui/button.png);
margin-top:60px;
margin-left:210px;
font-size:2em;
text-align:center;
color:white;

  }

<?php
   include("config.php");
   session_start();

   if($_SERVER["REQUEST_METHOD"] == "POST") {
      // username and password sent from form

      $myusername = mysqli_real_escape_string($db,$_POST['username']);
      $mypassword = mysqli_real_escape_string($db,$_POST['password']);

      $sql = "SELECT id FROM uzivatele WHERE login = '$myusername' and heslo = '$mypassword'";
      $result = mysqli_query($db,$sql);
      $row = mysqli_fetch_array($result,MYSQLI_ASSOC);

      $count = mysqli_num_rows($result);

      // If result matched $myusername and $mypassword, table row must be 1 row

      if($count == 1) {
         $_SESSION['login_user'] = $myusername;
             echo "    $('.left').animate({left: '250px'});";
      }else {
      }
   }
?>