As pointed out in the comments, you are including bottomBar.php before defining the variable $page. So, when bottomBar.php is being rendered, $page is still undefined!

Therefore, simply define $page before including bottomBar.php:

$page = 'index';
include("includes/bottomBar.php");

Solution: defining a public variable before including the bottomBar

the $page variable is defined after it’s used

 session_start();

  $page = 'index'; // definition

  include("includes/header.php");
  include("includes/bottomBar.php"); // Using

 include("includes/classes/User.php"); 
 include("includes/classes/Post.php");

I want my navbar icons change their colors when the user is on a certain page (like on instagram etc.) This is what I tried – bottomBar.php:

if ($page == 'friends') {
 $theme = 'link_friendsWall_active';
 $theme2 = 'link_index';
 $theme3 = 'link_trophypage';
 $theme4 = 'link_selfpage';
} else if ($page == 'index') {
 $theme = 'link_friendsWall';
 $theme2 = 'link_index_active';
 $theme3 = 'link_trophypage';
 $theme4 = 'link_selfpage';
} else if ($page == 'trophy') {
 $theme = 'link_friendsWall';
 $theme2 = 'link_index';
 $theme3 = 'link_trophypage_active';
 $theme4 = 'link_selfpage';
} else if ($page == 'profile') {
 $theme = 'link_friendsWall';
 $theme2 = 'link_index';
 $theme3 = 'link_trophypage';
 $theme4 = 'link_selfpage_active';
}

$page refers to the variables defined in files index.php (as index), friendsWall.php (as friends) etc. and the bottom_menu is included in them. The variables defined by the themes are classes that are styled in css. Then in my bottom_menu I’m calling those variables:

<div class="bottom_menu">


    <li><a class="<?=$theme?>" href="includes/handlers/friends_link.php">
            <i class="fas fa-user-friends"></i>
        </a></li>
    <li><a class="<?=$theme2?>" href="index.php">
            <i class="fas fa-users "></i>
        </a></li>
    <li><a href="#">
            <i class="fas fa-plus-circle"></i>
        </a></li>
    <li><a class="<?=$theme3?>" href="trophyPage.php">
            <i class="fas fa-trophy"></i>
        </a></li>
    <li><a class="<?=$theme4?>" href="myProfile.php">
            <img class='img_bottom_bar' src="<?php echo $user['profile_pic']; ?>" width="30px" height="30px" border-radius="20px">
        </a></li>


</div>

My problem is that the website doesn’t read the variable $page from other pages.
index.php:

  session_start();
  include("includes/header.php");
  include("includes/bottomBar.php");

 include("includes/classes/User.php");
 include("includes/classes/Post.php");

 $page = 'index';