Create a WordPress Plugin from Scratch

You can write your own plugin in minutes. A plugin is a folder (or even one PHP file) in wp-content/plugins. We will build a plugin that adds a notice box at the end of every post, step by step.

Step 1: Create the folder and file

Make a folder with a unique name, like tc-post-notice, and in it a PHP file with the same name. The comment at the top is the plugin header. Only Plugin Name is required, but add more details:

<?php
/**
 * Plugin Name: TC Post Notice
 * Description: Shows a notice box at the end of every single post.
 * Version:     1.0.0
 * Author:      Tutorials Class
 * License:     GPL-2.0-or-later
 * Text Domain: tc-post-notice
 */

// Stop if someone opens this file directly.
if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

/**
 * Add a notice after the content of a single post.
 */
function tcpn_add_notice( $content ) {
	if ( is_singular( 'post' ) && is_main_query() ) {
		$text    = get_option( 'tcpn_text', 'Thanks for reading!' );
		$content .= '<div class="tcpn-notice"><strong>' . esc_html( $text ) . '</strong></div>';
	}
	return $content;
}
add_filter( 'the_content', 'tcpn_add_notice' );

/**
 * Run once when the plugin is activated.
 */
function tcpn_activate() {
	add_option( 'tcpn_text', 'Thanks for reading!' );
}
register_activation_hook( __FILE__, 'tcpn_activate' );

Step 2: Activate and test

The plugin is visible in the list, it saved its option when it was activated, and the notice is on the post page.

What we used

• The ABSPATH check: stops direct access to the file from the browser.
• A prefix (tcpn_) for all functions and options, so they do not clash with other plugins.
• A filter (the_content): to change the post content.
• Escaping (esc_html): for safe output.
• An activation hook: for the setup work, like default options.

Deactivation and uninstall

Use register_deactivation_hook() to clean temporary data when the plugin is turned off, like scheduled events. When the plugin is deleted from the dashboard, WordPress looks for a file named uninstall.php in the plugin folder and runs it. (In WP-CLI, wp plugin uninstall does the same, while wp plugin delete only removes the files.) This is the right place to remove the options and tables of the plugin:

<?php
// Runs only when WordPress deletes the plugin.
if ( ! defined( 'WP_UNINSTALL_PLUGIN' ) ) {
	exit;
}
delete_option( 'tcpn_text' );

Good practice for plugin code

Rule	Why
Use a unique prefix for functions, classes, options and hooks	Avoid name clashes
Check ABSPATH at the top of every PHP file	Block direct access
Sanitize input, escape output, use nonces and check capabilities	Security. See Data Validation and Security
Use hooks, never edit the core	Safe updates
Load scripts only where needed	Speed
Write a readme.txt	Needed to publish on WordPress.org
Follow the WordPress Coding Standards	Clean and readable code

Organize bigger plugins

When the code grows, split it into files: an includes/ folder for classes, an admin/ folder for dashboard code, an assets/ folder for CSS, JS and images, and a languages/ folder for translations. The main file only loads the others with require_once plugin_dir_path( __FILE__ ) . 'includes/class-example.php';.

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