WordPress Dropins

WordPress dropins are likely a hidden feature of WordPress. Using dropins developers can replace, add, or enhance some advance core functionality of WordPress.
Even though Dropin Plugin are available on WordPress plugins page but they are much different from regular plugins. And we can not manage dropin directly from Plugin page unlike a regular plugin.

There are a total of 12 dropins in WordPress. These are single php files, we create in wp-content folder to achieve our desired goals.

Dropin File	Installation	Loading triggers	Description
db.php	Normal	On site load	To implement custom database in WordPress.
db-error.php	Normal	On a DB Error	To show custom alert message a DB error.
install.php	Normal	On WP Installation	To run own script on installion.
maintenance.php	Normal	On maintenance mode activation	To display custom maintenance message.
object-cache.php	Normal	On site load	To enable Redis object caching.
advanced-cache.php	Normal	If WP_CACHE is set as true	To enable advance caching in WordPress.
php-error.php	Normal	On a PHP error	To display custom PHP error message.
fatal-error-handler.php	Normal	On a Fatal error	To display custom message on PHP Fatal error.
sunrise.php	Multisite	If SUNRISE is set as true	To execute custom script before a multi-site is loaded.
blog-deleted.php	Multisite	On deleting a blog	To display a custom message on a sub-site deleted.
blog-inactive.php	Multisite	On a blog deactivation	To display custom blog deactivation message.
blog-suspended.php	Multisite	On archiving or spamming a blog	To display custom message on marking a blog as Archived or spam.

1. One of the most popular plugin Query Monitor uses db.php dropin to analyse the queries running a page load.
2. Most of caching plugins like W3 Total Cache, LiteSpeed Cache and WP Fastest Cache use the dropin ‘advanced-cache.php’ to implement their own caching mechanism in WordPress.
3. Redis Object Cache plugin uses the ‘object-cache.php’ dropin to implement object caching for speedup the php code execution.

To use a dropin we need to put the Dropin plugin PHP file into our wp-content folder openly without any folder. WordPress loads these unique named files at different triggers dynamically as mentioned in above table.

To achieve a custom domain mapping in a multisite network we have implemented the sunrise.php dropin.

<?php
/**
 * Domain mapping using sunrise dropins.
 *
 * We need to Map our domain from https://www.webkul.com/wordpress/ to https://www.webkul.com/web/wordpress/
 */

$extra_domains = array(
	'webkul.com/web/wordpress/' => 8, // 8 us the blog id of the subsite https://www.webkul.com/wordpress/
);
$server_uri    = $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'];

if ( isset( $_SERVER['REQUEST_URI'] ) && array_key_exists( $server_uri, $extra_domains ) ) {
	$mask_domain = $_SERVER['HTTP_HOST'] . '/web/wordpress/';

	// Set globals.
	$blog_id      = $extra_domains[ $mask_domain ]; // phpcs:ignore WordPress.WP.GlobalVariablesOverride.Prohibited
	$current_blog = get_site( $blog_id ); // phpcs:ignore WordPress.WP.GlobalVariablesOverride.Prohibited

	// This should always be 1, unless you are running multiple WordPress networks.
	$current_site = get_network( 1 ); // phpcs:ignore WordPress.WP.GlobalVariablesOverride.Prohibited

	$origin_domain = $current_blog->domain . untrailingslashit( $current_blog->path );

	add_filter(
		'home_url',
		function( $url ) use ( $mask_domain, $origin_domain ) {
			return str_replace( $origin_domain, $mask_domain, $url );
		}
	);
}

WordPress shows all the implemented dropins under the Plugins page. However we can not manage them for activation, deactivation directly from the plugin page unlike a regular plugin.

Dropins are Harmful for WordPress Dropins are very less explained (or Hidden) feature of WordPress that is why there are some misconception among WordPress developers and users. One of them is Dropins are harmful for WordPress, they might break the site and slow down the performace.

We have done so much R&D before writing this blog and din’t found any reason dropins are harmful for WordPres.

To Get all Dropins use function _get_dropins

Enable Advance Caching Dropin

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