A plugin in WordPress is a PHP script that extends or alters the core functionality of WordPress.

1. WordPress Plugins
By:- Mr. Ashok Negi

2. What is Plugin?
A plugin in WordPress is a PHP script that extends or alters the core functionality of
WordPress. Quite simply plugins are files installed in WordPress to add a feature, or
set of features, to WordPress
Plugin provides a set of hooks that enable plugins access to specific parts of
WordPress. WordPress contains two different types of hooks: Actions and Filters.
The Action hook enables you to trigger custom plugin code at specific points
during execution. For example, you can trigger a custom function to run after a
user registers a user account in WordPress.
The Filter hook to modifies text before adding or after retrieving from the database.

3. How to install any plugin in wordpress?
Following are the different ways to install any plugin
1: Unzip the zipped plugin and then copy and paste folder inside the plugin
folder the path to plugin folder is wp-contentplugins
2: Go to Admin > Plugins > Add New, You can search the plugin by keyword, author
or tag then you will get the list of plugin for the keyword you have entered.
3: The second link in Add New Plugin is “Upload” If you have a plugin in a .zip
format, you may install it by uploading it here.

4. How to create a custom plugin?
CREATING A PLUGIN FILE
A plugin in WordPress can be a single PHP file or a group of files inside a folder. You
need to consider many things when creating a new plugin in WordPress such as the
plugin name and proper folder usage.
NAMING YOUR PLUGIN
When choosing a name for your plugin, it’s good practice to consider a name based
on what your plugin actually does. For example, if you create an SEO - focused
plugin, you wouldn’t want to name it Bob’s Plugin. Your audience would have no
idea what your plugin actually does based on the plugin name. Your plugin name
should be unique to your plugin and should also be descriptive of your plugin’s
purpose.

5. How to create a custom plugin continue..
HEADER REQUIREMENTS
The plugin header is the only requirement for a plugin to function in WordPress. The
plugin header is a PHP comment block located at the top of your primary plugin PHP
file. This comment block tells WordPress that this is a valid WordPress plugin.
Following is an example of a plugin header:
< ?php
/*
Plugin Name: My Plugin
Plugin URI: http://example.com/wordpress-plugins/my-plugin
Description: A brief description of my plugin
Version: 1.0
Author: Opensourcetechnologies
Author URI: http://www.opensourcetechnologies.com
License: GPLv2
*/
?>

6. How to create a custom plugin continue ..
As you can see, the plugin header is straightforward. The only required line for
WordPress to recognize your plugin is the plugin name, but it’s good practice to fill in
the entire header as shown.

7. Hooks, Actions and Filters
Hooks are provided by WordPress to allow your plugin to 'hook into' the rest of WordPress;
that is, to call functions in your plugin at specific times, and thereby set your plugin in
motion. There are two kinds of hooks:
1. Actions: Actions are triggered by specific events that take place in WordPress, such as
publishing a post, changing themes, or displaying a page of the admin panel.
You can check all actions list at http://codex.wordpress.org/Plugin_API/Action_Reference
Common actions functions are has_action(), add_action(), do_action(),
do_action_ref_array(), did_action(), remove_action(), remove_all_actions()
2. Filters: Filters are the hooks that WordPress launches to modify text of various types
before adding it to the database or sending it to the browser screen. Your plugin can specify
that one or more of its PHP functions is executed to modify specific types of text at these
times, using the Filter API.
You can check all filters list at: http://codex.wordpress.org/Plugin_API/Filter_Reference
Common filter funtions are has_filter(), add_filter(), apply_filters(), current_filter(),
merge_filters(), remove_filter(), remove_all_filters()

8. Example Action
To email some friends whenever an entry is posted on your blog
function email_friends( $post_ID ) {
$friends = 'bob@example.org, susie@example.org'; wp_mail( $friends, "sally's blog
updated", 'I just put something on my blog: http://blog.example.com' ); return
$post_ID;
}
add_action('publish_post', 'email_friends');
publish_post: This action hook runs when a post is published, or if it is edited and its
status is "published". Action function arguments: post ID
email_friends: This is the function, get called on publishing a post and accepting a
parameter post ID.

9. Example Filter
This filter function adds an image before the post on the post page. It assumes an
image named post_icon.png exists in the theme images folder. It runs at a lower
priority (20) which runs later than most other filters (default is 10).
function my_the_content_filter( $content ) {
if ( is_single() )
// Add image to the beginning of each page
$content = sprintf( '<img class="post-icon" src="%s/images/post_icon.png" alt="Post
icon" title=""/>%s', get_bloginfo( 'stylesheet_directory' ), $content );
// Returns the content.
return $content;
}
add_filter( 'the_content', 'my_the_content_filter', 20 );

10. Example Plugins
Now I am going to show three sample plugin that I have made to make you more
clear about the plugin
-Helloworld : In this example I have created very simple function call and no hook.
-simple-add-to-footer: In this example I have used two hooks ‘wp_footer’ and
‘the_content’.
-Myplugin: In this example I have used two hooks ‘the_content’ and ‘the_title’

11. Thanks