Advanced WordPress Series-1: Filter

Plugin: To create a plugin we should create a directory in the wp folder and create a .php file in that dir. The plugin detail needs to be written inside a comment block to make this visible to wp.

Filter Hooks: We use add_filter() & remove_filter() to modify content i.e. text. Filter is a way to modify the post content. Two simple filter hook, first one makes first letter of each words of title to uppercase; second one adds a timestamp at the end of content of posts.

<?php

/* 
 * Plugin Name: sajib filter
 * Plugin URI: net.sajibbiswas.com
 * Description: just demo
 * Author: sajib
 * Author URI: sajib.com
 * Version: 1.0
 */

add_filter('the_title', ucwords);

add_filter('the_content', function($content){
	return $content . '<br>' .time();
});

Action Hooks:  We use actions, add_actions() & remove_actions() to hook into specific events that occurs in the lifecycle of wp. The first function of the following codeblock echoes a sentence in the footer area and the second one  sends an email each time a new comment is left on the blog.

add_action('wp_footer', function(){
	echo "hello, from the footer.";
});
 

add_action('comment_post', function(){
	$email = get_bloginfo("admin_email");
	wp_mail(
			$email,
			'new comment posted',
			'A new comment has been left on your blog'
	);
});

A simple plugin that adds 3 recent post’s link at the end of the content.

add_filter('the_content', function($content){
	if( !is_singular('post') ){
		return $content;
	}
	
	$terms = get_the_terms( the_ID(), "category");
	$cats = array();
	foreach( $terms as $term){
		$cats[] = $term->term_id;
	}
	
	$loop = new WP_Query( array(
		'posts_per_page' => 3,
		'category__in' => $cats,
		'orderby' => 'rand',
		'post__not_in' => array( get_the_ID() ),
	));
	
	if( $loop->have_posts() ){
		$content .= '<h2>You might also like....</h2>'
				. '<ul class="related-cat-posts">';
		
		while( $loop->have_posts() ): $loop->the_post();
			$content .= '<li><a href="'. get_permalink() .'">'. get_the_title() .'</a></li>';
		endwhile;
		
		$content .= '</ul>'; 
		wp_reset_query();
	}
	
	return $content;
});