In this chapter, we step through the plugins’ main file. We also explain the benefits of PHP Classes to code the plugin instead of functions.

Plugin Info

The main file share-on-social.php starts with header comments. As already described in the Section 2.1, “WordPress Simple Plugin”](wordpress-simple-plugin/), WordPress uses the header comments from the main file to display the Plugin info in plugins page.

Define Constants

Throughout the plugin, we often refer some paths and constants. Normally, the constants are defined at the start of plugins. In Share on Social, we define following constants.


define( 'SOS_NAME', 'share-on-social' );
define( "SOS_VERSION", '1.0.0' );

define( "SOS_URL", trailingslashit( plugin_dir_url( __FILE__ ) ) );

// plugin path
define( "SOS_PATH", plugin_dir_path( __FILE__ ) );

//basename share-on-social/share-on-social.php
define( "SOS_PLUGIN_BASENAME", plugin_basename( __FILE__ ) );

// main file - share-on-social.php
define( "SOS_PLUGIN_FILE", __FILE__ );

We prefix constants with SOS_ so that names don’t clash with constants from theme and other plugins. Always prefix the constants with the plugin name or some unique abbreviation.

Functions and Classes

WordPress Plugin Main File and Class files.

It is easy to code the plugin functionality with PHP functions. But this approach may results in name conflict either with functions defined by site theme and other active plugins or even with WordPress API.

Even though bit difficult to read, better approach is to use PHP Classes which eliminates the name conflict.

In Share on Social Plugin, we use classes for all modules except the main file and uninstall.php where we use regular functions. Rest of the code is split into class files which are included in main file.

Let’s see how this is done with two class files; one for front end and another for admin module. In the following code snippet, we include two class files class-frontend.php and class-admin.php.


// plugin entry point

function setup_sos_plugin () {

    require_once SOS_PATH . 'include/class-helper.php';
    if ( is_admin() ) {
        require_once SOS_PATH . 'admin/class-admin.php';
        add_action( 'plugins_loaded', 'load_sos_admin' );

    } else {

        require_once SOS_PATH . 'frontend/class-frontend.php';
        add_action( 'plugins_loaded', 'load_sos_frontend' );


function load_sos_admin () {
    $sos_admin = new Sos_Admin();

function load_sos_frontend () {
    $sos_frontend = new Sos_Frontend();

It is important to note that load_sos_admin() and load_sos_frontend() are not called immediately by add_action() in setup_sos_plugin(), but their execution is deferred.


Just for the sake of explanation, we split this into two phases. When we access the site, as part of phase one, WordPress processes all activated plugins. When share-on-social.php is processed its setup_sos_plugin() function is called. In this function, WordPress includes the class files and calls add_action() which just registers load_sos_admin() or load_sos_frontend() as callback functions with WordPress for a hook named plugins_loaded. Then WordPress goes on to process other plugins which are active in the site. When all active plugins are processed, WordPress triggers second phase by calling the hook plugins_loaded. At this point, WordPress calls registered callback methods load_sos_admin() or load_sos_frontend() and executes the code they contain.

Next, let’s go through a plugin class file to see how classes are defined with a snippet from class-frontend.php .



defined( 'ABSPATH' ) or die( "Access denied !" );

class Sos_Frontend {

    var $sos_shortcode = 'share-on-social';

    public function setup () {
        // enable shortcode
        add_shortcode( $this->sos_shortcode, 
                ) );

    // other methods of the class


In the class file, we define class Sos_Frontend and its method. At the top, we define variables which have class scope such as $sos_shortcode. In classes, we use setup() to carry out initializations tasks such as add actions, filters, shortcodes and also to include other class files and modules etc., Other class files of the plugin follow similar pattern. For the moment, it is enough to understand the overall structure of the plugin and its files as throughout the tutorial, we cover its code in detail.

In the next chapter, we explain the the important WordPress concepts Actions, Filters and Hooks which allows plugins to plug to the WordPress core.