What is the WordPress Plugin Hello Dolly

Hello Dolly is a plugin that comes pre-installed with WordPress, and it displays a random excerpt from the lyrics of Louis Armstrong’s song on the administration screen.

The description of the WordPress plugin is as follows:

This is not just a plugin, it symbolizes the hope and enthusiasm of an entire generation summed up in two words sung most famously by Louis Armstrong: Hello, Dolly. When activated you will randomly see a lyric from Hello, Dolly in the upper right of your admin screen on every page.

Understanding the Behavior from the Source Code of “Hello Dolly”

There is only one PHP file used in “Hello Dolly.” Let’s read it to learn more about the plugin.

The beginning of the file contains comments with Plugin Name, Plugin URI, Description, Author, Version, and Author URI. At least Plugin Name is required for the plugin.

Within this file, there are three functions: hello_dolly_get_lyric, hello_dolly, and dolly_css.

hello_dolly_get_lyric

This function selects and returns one line from the lyrics of “Hello Dolly.” It uses mt_rand to get a random index. The function wptexturize is used to format the text according to WordPress standards and is still available in WordPress 5.

The lyrics contain the line “Dolly’ll never go away” twice, and there are other duplicate lines as well. While mt_rand outputs a random line, it does not remove duplicates, so those duplicate lines have a higher chance of being selected.

hello_dolly

This function retrieves one line from hello_dolly_get_lyric, converts it into HTML, and displays it. The returned HTML is shown on WordPress pages within a p tag with an id attribute of dolly.

hello_css

This function displays the CSS applied to the displayed message as the content of the style tag. The function is_rtl determines whether the language used is right-to-left or left-to-right, adjusting the display accordingly.

In addition to these function definitions, two actions are added within the file.

add_action('admin_notices', 'hello_dolly');

This specifies that the hello_dolly function should be executed on the action event admin_notice. The admin_notice refers to the timing of displaying notifications in the admin area, including messages such as “Plugin activated.”

add_action('admin_head', 'dolly_css');

This specifies that the dolly_css function should be executed on the action event admin_head. The admin_head is an event that occurs during the generation of the head tag when opening the admin area, and any text output here will be placed inside the head tag.

Extending the Plugin

Now that we understand how to use a simple plugin, let’s think about modifying the code to create a new plugin.

For instance, it would be interesting to display messages from “Mai-ni-chi, Shuzo! Kokoro o genki ni suru honki no ouen messeiji” based on the current date.

To obtain the date in PHP, you can use the following code to get the date in the Tokyo timezone.