<nav class="navbar navbar-expand-lg navbar-light bg-light">
    <?php if($_SESSION["login_state"]=="1"){ ?>
    <a class="navbar-brand" href="index.php?site=blog">Blog</a><?php }
          else
          { ?>
    <a class="navbar-brand" href="index.php">Blog</a><?php } ?>
    <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
        <span class="navbar-toggler-icon"></span>
    </button>

    <div class="collapse navbar-collapse" id="navbarSupportedContent">
        <?php if($_SESSION["login_state"]=="1"){ ?>
        <ul class="navbar-nav mr-auto">
            <li class="nav-item">
                <a class="nav-link" href="index.php?site=blog">Blog list</a>
            </li>
            <li class="nav-item">
                <a class="nav-link" href="index.php?site=blog&action=adatfelvetel_form">Add new post</a>
            </li>

        </ul>
        <label class="my-2 my-lg-0">
            Welcome, <?php echo $_SESSION["user_name"] ?>
        </label>

        <a class="nav-link" href="index.php?action=logout">Logout</a><?php }
              else
              { ?>
        <a class="nav-link" href="index.php?action=login">Login</a><?php } ?>
    </div>
</nav>