Plugin Workshop actions & filters demo plugin

demo.php

<?php
/*
 * Plugin Name: Workshop Demo Plugin
 */

// Action example
function demo_add_html_to_footer() {
	echo "<h1 style='color:white;'>I'm an example!</h1>";
}
// add_action( 'wp_footer', 'demo_add_html_to_footer' );

// ------------------------------------------------------------

// Filter example
function demo_enhance_description( $value ){
	// In pre_option_blogdescription, $value is empty, and we're preempting the database read
	// return 'A new site description';
	// In option_blogdescription, we have the site description in $value
	return $value . '!';
}
// add_filter( 'pre_option_blogdescription', 'demo_enhance_description' );
// add_filter( 'option_blogdescription', 'demo_enhance_description' );

// ------------------------------------------------------------

// Action with argument
function demo_restrict_homepage_category( $query ) {
	// Check out WP_Query: https://codex.wordpress.org/Class_Reference/WP_Query
	if ( $query->is_home() && $query->is_main_query() ) {
		$query->set( 'category_name', 'travelling' );
	}
}
// add_action( 'pre_get_posts', 'demo_restrict_homepage_category' );

// ------------------------------------------------------------

// Filter with multiple arguments
function demo_add_id_to_title( $title, $id ) {
	// Formatting function
	// return sprintf( '%s [%s]', $title, $id );

	// Plain string concatenation
	return $title . ' ['. $id . ']';
}
// add_filter( 'the_title', 'demo_add_id_to_title', 10, 2 );

// ------------------------------------------------------------

// All: display all hooks run on a page
function demo_display_all () {
	if ( ! in_array( current_filter(), [ 'gettext_with_context', 'gettext' ] ) ) {
		var_dump( current_filter() );
	}
}
// add_action( 'all', 'demo_display_all' );