Only the category ID is given. So to get the category, you'd need a mysql query:

$query = "SELECT title FROM #__categories WHERE id = ".intval($article->catid)." LIMIT 1";
$database->setQuery($query);
$category = $database->loadResult();

The variable $category should now have the category title so the text looks like.

$custom_text = 'this '.$article->title.' belongs to '.$category;

Where should i post this code?

$query = "SELECT title FROM #__categories WHERE id = ".intval($article->catid)." LIMIT 1";  
$database->setQuery($query);  
$category = $database->loadResult();

knock knock..

Anywhere above:

$custom_text = 'this '.$article->title.' belongs to '.$category;

i saw this message

Parse error: syntax error, unexpected '=' in /home/p0o9ip0o/public_html/plugins/content/botphp_content.php(53) : eval()'d code on line 6

i used this code

<?php $query = "SELECT title FROM #__categories WHERE id = ".intval($article->catid)." LIMIT 1";  
$database->setQuery($query);  
$category = $database->loadResult();  
custom_text = 'this '.$article->title.' belongs to '.$category;
$article->text = str_replace('{additionaltext}', $custom_text, $article->text); ?>

This line:

custom_text = 'this '.$article->title.' belongs to '.$category;

Should be:

$custom_text = 'this '.$article->title.' belongs to '.$category;

Variables in PHP must begin with "$".