Displaying HTML based on the selection - php

Question

I'm doing this PHP tutorial on OOP and I'm trying to incorporate the procedural code from the essential training into it, to see if I actually learned anything beyond the basics, and I got stuck, big time.
So, I have these two functions inside the class I named Blog:
public function navigation($selected_subject, $selected_page) {
$output = "<ul class=\"subjects\" id=\"accordion\">";
$subject_set = $this->get_all_subjects($public);
while ($subject = mysqli_fetch_array($subject_set)) {
$output .= "<li";
if ($subject["id"] == $selected_subject['id']) { $output .= " class=\"blogselected\""; }
$output .= ">{$subject["menu_name"]}</li>";
$page_set = $this->get_pages_for_subject($subject["id"], $public);
$output .= "<ul class=\"pages\">";
while ($page = mysqli_fetch_array($page_set)) {
$output .= "<li";
if ($page['id'] == $selected_page['id'] ) { $output .= " class=\"blogselected\""; }
$output .= "><a href=\"../public/blog_index.php?page=" . urlencode($page["id"]) .
"\">{$page["menu_name"]}</a></li>";
}
$output .= "</ul>";
}
$output .= "</ul>";
return $output;
}
public function blog_page($selected_page){
if ($selected_page) {
echo "<h2>" . $selected_page['menu_name'] . "</h2>";
echo "<div class=\"page-content\">";
echo $selected_page['content'];
echo "</div>";
} else {
echo "<h2>Welcome to Widget Corp</h2>";
}
}
and this piece of code on the index page:
<?php
$blog = new Blog;
global $selected_page;
global $selected_subject;
?>
<div class="blog">
<div class="blogpost">
<?php echo $blog->blog_page($selected_page); ?>
</div>
<div class="blognav">
<?php echo $blog->navigation($selected_subject, $selected_page); ?>
</div>
</div>
The code partially works. The navigation with subjects and sub-pages gets displayed fine, when the links are clicked the id "?page=1" is passed to the url, but the problem is that the code is not adding a class "blogselected" to the active links, and it also does not display the content when the links are clicked. The only thing that gets displayed in the content area is the h2 "Welcome to Widget Corp" from else condition in the second function, so I believe the variables $selected_subject and $selected_page might be causing the problem, but I just can't figure it out on my own. I spent countless hours trying to solve this and I'm on the brink of giving up completely. What exactly am I missing here?