Short Description

WordPress is prefered CMS for sites with Pages and Blogs

Detailed Description

Types of extensions possible

WordPress extensions are called plugins.
Plugins can

1. Show Custom Functionalities in Pages and Blogs. See add_shortcode below
2. Extend/Replace Core funtionalities with hooks

How system identifies extension

Header Requirements

/**
 * Plugin Name
 *
 * @package           PluginPackage
 * @author            Your Name
 * @copyright         2019 Your Name or Company Name
 * @license           GPL-2.0-or-later
 *
 * @wordpress-plugin
 * Plugin Name:       Plugin Name
 * Plugin URI:        https://example.com/plugin-name
 * Description:       Description of the plugin.
 * Version:           1.0.0
 * Requires at least: 5.2
 * Requires PHP:      7.2
 * Author:            Your Name
 * Author URI:        https://example.com
 * Text Domain:       plugin-slug
 * License:           GPL v2 or later
 * License URI:       http://www.gnu.org/licenses/gpl-2.0.txt
 * Update URI:        https://example.com/my-plugin/
 */
 // short code for this plugin will be 'osolwpccc'.This line is not mandatory but useful for future reference for developers , while modifying the plugin

Additionally contents of readme.txt is also important. Espescially Stable tag:

Sample Readme Contents

Frontend

Detecting activation

register_activation_hook and register_deactivation_hook
register_activation_hook

/* Main Plugin File */
function my_plugin_activate() {
  add_option( 'Activated_Plugin', 'Plugin-Slug' );
  /* activation code here */
}
register_activation_hook( __FILE__, 'my_plugin_activate' );
//-----------------------------------------------
function load_plugin() {
    if ( is_admin() && get_option( 'Activated_Plugin' ) == 'Plugin-Slug' ) {
        delete_option( 'Activated_Plugin' );
        /* do stuff once right after activation */
        // example: add_action( 'init', 'my_init_function' );
    }
}
add_action( 'admin_init', 'load_plugin' );

Interaction with system

1. add_action and add_filter

An add_action in WordPress is what you use to create a trigger hook. When something happens, then do-something-else.

An add_filter is used to hook data, i.e. change/replace. For example, where there is [some-code], change it to some-other-expanded-code.

Hooks are handled in wp-includes\class-wp-hook.php

eg:

2. add_action('init',
3. add_shortcode
4. wp_enqueue_script and wp_enqueue_style to load scripts
   1. the wp_head, wp_footer,admin_head,admin_footer,login_head and login_footer action hooks
   2. the wp_enqueue_scripts , admin_enqueue_scripts , and login_enqueue_scripts action hooks
   3. the wp_enqueue_script() WordPress function

function cccontact_jquery_enqueue() {

       wp_enqueue_script('jquery');
       //also initiate thick box
        if(!is_admin()) // thickbox wont load in admin
        {
            //wp_enqueue_script('jquery');
            wp_enqueue_script('thickbox',null,array('jquery'));
            wp_enqueue_style('thickbox.css', '/'.WPINC.'/js/thickbox/thickbox.css', null, '1.0');
        }
    }
add_action("wp_enqueue_scripts", "cccontact_jquery_enqueue", 11);   

5. Adding custom JS or CSS to head tag

   function osolwpccc_custom_javascript() {
   ?>
       <script>
         // your javscript code goes here
         console.log('osolwpccc_custom_javascript')
       </script>
   <?php
   }
   add_action('wp_head', 'osolwpccc_custom_javascript');

6. Develelopment Prop

// see all functions hooked to 'wp_footer'
add_action('wp', function () {
    echo '<pre>';
    print_r($GLOBALS['wp_filter']['wp_footer']);
    echo '</pre>';
    exit;
});

Accessing REQUEST and Session Vars

Usual PHP $_REQUEST and $_SESSION vars

Display Errors

WP_Error class

$errors = new WP_Error();
$errors->add('error code', 'detailed error');

System generated Errors are available in add_filter functions, $errors is passed as an argument, which should be returned back return($errors);

Handling AJAX

reference

1. wpajax$action
2. wp_ajaxnopriv$action

Example

//***********************************************display captcha hooks 
// depending on wether logged in or not, the following hooks gets triggered when calling url contains action=cccontact_cccontact_display_captcha
add_action('wp_ajax_cccontact_display_captcha', [$OSOLCCC_CommonClass_inst,'cust_captcha_display_captcha']);// executed when logged in
add_action('wp_ajax_nopriv_cccontact_display_captcha', [$OSOLCCC_CommonClass_inst,'cust_captcha_display_captcha']);// executed when logged out

Using DB

refer

DB Class: $wpdb/$GLOBALS['wpdb']
DB Class File: wp-includes/wp-db.php
Table Prefix: $wpdb->prefix(prefix to table names).
Last Insert Id: $wpdb->insert_id
Affected Rows: $wpdb->rows_affected

Main Methods:

1. $wpdb->get_results()
2. $wpdb->query()
3. $wpdb->prepare()
4. Display Error

   <?php 
   define( 'DIEONDBERROR', true );// put this in wp-config.php
   $wpdb->print_error(); 
   ?>

Example Usages

$results = $GLOBALS['wpdb']->get_results( query, output_type );

Where output_type is
One of three pre-defined constants. Defaults to OBJECT.

1. OBJECT – result will be output as an object.
2. ARRAY_A – result will be output as an associative array.
3. ARRAY_N – result will be output as a numerically indexed array.

   // 1st Method - Declaring $wpdb as global and using it to execute an SQL query statement that returns a PHP object
   global $wpdb;
   $results = $wpdb->get_results( "SELECT * FROM {$wpdb->prefix}options WHERE option_id = 1", OBJECT );

   OR

   <?php
   // 2nd Method - Utilizing the $GLOBALS superglobal. Does not require global keyword ( but may not be best practice )
   $results = $GLOBALS['wpdb']->get_results( "SELECT * FROM {$wpdb->prefix}options WHERE option_id = 1", OBJECT );

   Prepared Statements

   <?php
   $metakey   = 'Funny Phrases';
   $metavalue = "WordPress' database interface is like Sunday Morning: Easy.";
   $wpdb->query(
   $wpdb->prepare(
     "INSERT INTO $wpdb->postmeta
     ( post_id, meta_key, meta_value )
     VALUES ( %d, %s, %s )",
     10,
     $metakey,
     $metavalue
   )
   );

   The same query using vsprintf()-like syntax. Note that in this example we pack the values together in an array. This can be useful when we don’t know the number of arguments we need to pass until runtime.

   <?php
   $metakey   = 'Funny Phrases';
   $metavalue = "WordPress' database interface is like Sunday Morning: Easy.";
   $wpdb->query(
   $wpdb->prepare(
   "INSERT INTO $wpdb->postmeta
   ( post_id, meta_key, meta_value )
   VALUES ( %d, %s, %s )",
   array(
        10,
        $metakey,
        $metavalue,
     )
   )
   );

   Select a variable

<?php $wpdb->get_var( 'query', column_offset, row_offset ); ?> 

eg:

<?php
$user_count = $wpdb->get_var( "SELECT COUNT(*) FROM $wpdb->users" );
echo "<p>User count is {$user_count}</p>";
?>

Get Row

<?php $wpdb->get_row('query', output_type, row_offset); ?> 

Get Col

get_col( 'query', column_offset ); ?>

Getting User Details

refer
get_userdata( int $user_id )
Example

<?php $user_info = get_userdata(1);
      echo 'Username: ' . $user_info->user_login . "\n";
      echo 'User roles: ' . implode(', ', $user_info->roles) . "\n";
      echo 'User ID: ' . $user_info->ID . "\n";
?>

Another one

<?php $user_info = get_userdata(1);
      $username = $user_info->user_login;
      $first_name = $user_info->first_name;
      $last_name = $user_info->last_name;
      echo "$first_name $last_name logs into her WordPress site with the user name of $username.";
      $display_name = $user_info->display_name;
      $user_email = $user_info->user_email;
?>

Pagination

refer
In posts use the tag <!--nextpage-->
Incorporating Pagination in Template

<?php if ( have_posts() ) : ?> 
    <!-- Add the pagination functions here. --> 
    <!-- Start of the main loop. -->
    <?php while ( have_posts() ) : the_post(); ?> 
    <!-- the rest of your theme's main loop --> 
    <?php endwhile; ?>
    <!-- End of the main loop --> 
    <!-- Add the pagination functions here. --> 
<div class="nav-previous alignleft"><?php next_posts_link( 'Older posts' ); ?></div> 
<div class="nav-next alignright"><?php previous_posts_link( 'Newer posts' ); ?></div> 
<?php else : ?>   
<?php _e('Sorry, no posts matched your criteria.'); ?> 
<?php endif; ?>

Mailing

wp_mail function.

Javascript builtin options availavle

1. Show Modal

Relative URLs

1. plugins_url()

Multilanguage implementation

Call hook load_plugin_textdomain for internationalising. refer

load_plugin_textdomain('your-plugin-short-code', false, dirname( plugin_basename(__FILE__)).'/languages');

For more info refer
Wordpress uses gettext Library
Language files must be saved as .mo (Machine Object)files , create .mo files from .po files with poedit, .pot is PO Template files( POT files are the template files for PO files. They will have all the translation strings left empty). So the workflow is POT (original strings) to PO (original strings and their translation to a specific language) to MO (compiled binary result)
texts should be called like echo __( 'WordPress is the best!', 'my-plugin' );
OR

printf(         
            __( 'Your city is %s.', 'my-plugin' ),
            $city
        );

If you have more than one placeholder in a string, it is recommended that you use argument swapping. In this case, single quotes (') around the string are mandatory because double quotes (") will tell php to interpret the $s as the s variable, which is not what we want.

printf(
            // translators: 1: Name of a city 2: ZIP code 
            __( 'Your city is %1$s, and your zip code is %2$s.', 'my-plugin' ),
            $city,
            $zipcode
        );

Adding 'adsense'

Simplest method is to use Head, Footer and Post Injections plugin.
To install,go to Plugins → Add New from your admin dashboard, then searching for Head, Footer and Post Injections.

After you install

1. Get your adsense id from your adsense account.
2. Settings → Head & Footer Code
3. Add the following code in footer

<SCRIPT src="js/tagdiv_theme.js" type="text/javascript"></SCRIPT>
<script src="http://www.google-analytics.com/urchin.js" type="text/javascript"></script>
<script type="text/javascript">
_uacct = "UA-1565652-1";
urchinTracker();
</script>

Backend

Adding configuration pages

1. / Hook to initalize the admin menu /

   add_action('admin_menu',

Additional options to interact with plugin

Identifying User Levels

1. is_admin()
2. is_super_admin()

Crons

refer
With WP-Cron, all scheduled tasks are put into a queue and will run at the next opportunity (meaning the next page load). So while you can’t be 100% sure when your task will run, you can be 100% sure that it will run eventually.

To setup cron jobs , refer

To prevent WP-Cron on each page load.In wp-config.php.

define('DISABLE_WP_CRON', true);

Adding the Hook
refer
In order to get your task to run you must create your own custom hook and give that hook the name of a function to execute. This is a very important step. Forget it and your task will never run.
eg:Remember, the bl_ part of the function name is a function prefix(to uniquely identify our plugin's cron tasks ). similar to wp_ajax_$action

add_action( 'bl_cron_hook', 'bl_cron_exec' );

and schedule cron refer

0 0 * * * wget --delete-after http://YOUR_SITE_URL/wp-cron.php

Disable from DB

Use phpMyAdmin to deactivate all plugins with the following steps.

1. In the table wp_options, under the option_name column (field) find the active_plugins row
2. Change the option_value field to: a:0:{}

Disabling extensions without removing settings

1. Rename plugins folder to plugins.hold
2. Login to Admin >> Plugins . This will disable all plugins.

Show "Briefly unavailable for scheduled maintenance. Check back in a minute"

Place a file named .maintenance in the blog base folder (folder that contains the wp-admin folder)

Debugging

Debugging

in wp-config.php, comment define( 'WP_DEBUG', false );

and add

if(!defined('WP_DEBUG'))
{
    // Enable WP_DEBUG mode
    define( 'WP_DEBUG', true );
    // Enable Debug logging to the /wp-content/debug.log file
    define( 'WP_DEBUG_LOG', true );
    // Disable display of errors and warnings
    define( 'WP_DEBUG_DISPLAY', false );
    @ini_set( 'display_errors', 0 );
    // Use dev versions of core JS and CSS files (only needed if you are modifying these core files)
    define( 'SCRIPT_DEBUG', true );
}

Troubleshooting

Troubleshooting

Templating

Extension Developer FAQs

https://developer.wordpress.org/plugins/wordpress-org/plugin-developer-faq/

Submitting extension

http://codex.wordpress.org/Plugin_Submission_and_Promotion
submit page
http://wordpress.org/plugins/add/

You will get an email from the wordpress term after they review it .
Content would be like.

---

osolwordpress,

Your plugin hosting request has been approved.

Within one hour, you will have access to your SVN repository at

http://plugins.svn.wordpress.org/customizable-captcha-and-contact-us-form/

with your WordPress.org username and password (the same one you use on the forums).

Here's some handy links to help you get started.

Using Subversion with the WordPress Plugins Directory
http://wordpress.org/plugins/about/svn/

FAQ about the WordPress Plugins Directory
http://wordpress.org/plugins/about/faq/

WordPress Plugins Directory readme.txt standard
http://wordpress.org/plugins/about/readme.txt

readme.txt validator:
http://wordpress.org/plugins/about/validator/

Enjoy!

---

Publishing

Publish after SVN access is got
http://eamann.com/tech/how-to-publish-a-wordpress-plugin-subversion/
https://in.godaddy.com/blog/publish-first-wordpress-plugin/

eg:
http://plugins.svn.wordpress.org/customizable-captcha-and-contact-us-form

of the plugin https://wordpress.org/plugins/customizable-captcha-and-contact-us-form/

Updating extension

1. Add plugin files to new tag directory : Once you’ve added all of your directories and files for your plugin within the trunk directory, you’ll then want to add them to a new tag directory with the new version as the folder name.

2. In readme.txt , edit Stable tag: and set to the tag of the latest version in svn