internetprogrammierung/frontend/segments/_header.php

<div>
  <div id="carouselExampleSlidesOnly" class="carousel slide"  data-ride="carousel">
    <div class="carousel-inner" style="height: 100px;">
      <div class="carousel-item active">
        <img class="d-block h-100" src="img/header_pink.png" alt="First slide">
      </div>
      <div class="carousel-item">
        <img class="d-block h-100" src="img/header_lime.png" alt="Second slide">
      </div>
      <div class="carousel-item">
        <img class="d-block h-100" src="img/header_blue.png" alt="Third slide">
      </div>
    </div>
  </div>
</div>

<!--Navbar-->
<nav class="navbar sticky-top navbar-expand-lg navbar-dark bg-dark">
  <a class="navbar-brand" href="index.php?page=home">FriendsBook</a>
  <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">
    <ul class="navbar-nav mr-auto">
      <li id="navItemHome" class="nav-item">
        <a class="nav-link" href="index.php?page=home">Home <span class="sr-only">(current)</span></a>
      </li>
      <li id="navItemEntry" class="nav-item">
        <a class="nav-link" href="index.php?page=newEntry">New Entry</a>
      </li>
    </ul>
    <form class="form-inline my-2 my-lg-0">
      <div class="btn-group" role="group" aria-label="Basic example">
        <!-- Button Login -->
        <a class="btn btn-primary" href="index.php?page=login" role="button">
          Login
        </a>

        <!-- Button Register -->
        <a class="btn btn-secondary" href="index.php?page=register" role="button">
          Register
        </a>
      </div>
    </form>
  </div>
</nav>