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you CAN configure the web server to treat .html (or any other extension) as a php file. the default is for only the .php extension.

UPDATE / DELETE queries don't have a result set, as such mysqli_query will just return true/false to indicate whether the query was successful or not. There's nothing to fetch so no reason to use mysqli_fetch_array. By returning the result, I mean those methods could have just been: public function deleteCategory($id) { return mysqli_query($this->conn, "DELETE FROM top_categories WHERE top_cat_id=$id"); } mysqli_query will return true or false. Adding the variable and if branch to check if it's true, just to then return true if it is, is entirely unnecessary.

Your file should be named with a .php extension, not .html. You need to load the file via the web server, not by just double-clicking on it to open it in windows. Open your browser and load the url http://localhost/index.php

Your file name is index.html and not index.php. You need a php file for php code to be executed.