navbar.php

<nav class="navbar navbar-expand-lg navbar-dark bg-primary">
  <a class="navbar-brand" href="#">Blog</a>
  <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarColor01" aria-controls="navbarColor01" aria-expanded="false" aria-label="Toggle navigation">
    <span class="navbar-toggler-icon"></span>
  </button>

  <div class="collapse navbar-collapse" id="navbarColor01">
    <ul class="navbar-nav mr-auto">
      <li class="nav-item active">
        <a class="nav-link" href="<?php echo ROOT_URL.'main.php'; ?>">Home
          <span class="sr-only">(current)</span>
        </a>
         <a class="nav-link" href="<?php echo ROOT_URL.'addpost.php'; ?>">Add Post
          
        </a>
      </li> 
  </div>
</nav>