WordPress: Create custom filter

vh_filter.php

<?php
/*
Plugin Name: VH Filter
Plugin URI: http://vienhoang.com
Description: Filter the content.
Version: 1.0
Author: Vien Hoang
Author URI: http://vienhoang.com
License: GPL2
*/

// Add filter is use to filter content
add_filter( 'the_title' , ucwords );

// add_filter( 'the_content', function($content) {
// 	return $content . ' ' . time();
// } );


// add_action( 'wp_footer', function() {
// 	echo 'Hello footer';
// } );

add_action( 'comment_post', function() {
	$email = get_bloginfo( 'admin_email' );
	// WP email function
	wp_mail(
		$email,
		'New Comment Posted',
		'A new comment has been posted.'
	);
} );

add_filter( 'the_content', function( $content ) {
	$id = get_the_id();
	if ( ! is_singular( 'post' ) ) {
		return $content;
	}
	// Retrieve the terms of the taxonomy that are attached to the post.
	$terms = get_the_terms( $id, 'category' );
	$cats = array();
	// Loop throught the terms array with objects and fetch the category id
	foreach ( $terms as $term) {
		$cats[] = $term->cat_ID;
	}


	$loop = new WP_Query(
		array(
			'post_per_page' => 3,
			'category__in' => $cats,
			'orderby' => 'rand',
			// Not fetch current post
			'post__not_in' => array($id)
		)
	);

	if ( $loop->have_posts() ) {
		$content .= '
			<h2>You Also Might Like.. </h2>
			<ul class="related-category-post">';
		
		while( $loop->have_posts() ) {
			$loop->the_post();

			$content .= '
			<li>
				<a href="' . get_permalink() .'">' . get_the_title() .'</a>
			</li>';
		}

		$content .= '</ul>';
		wp_reset_query();
	}

	return $content;
} );