Greg Winiarski

BlackBox is symfony like unobstrusive debug bar attached to the top of the browser window.
How can it help you with development: 
1. instantly inspect global variables (GET, POST, COOKIE, SERVER) 
2. debug both frontend and admin area 
3. executed MySQL queries and time it took to execute each query (useful for finding slow queries) 
4. profiler for measuring performance of your plugins and themes 
5. errors occurred when loading WordPress page

BlackBox Debug Bar

Immediate feedback on a great majority of errors. Added a conditional red background to the Errors, and with this modification I have never miss even a notice anymore.

Thank you very much!

Essential plugin in my developer toolset

BlackBox is a plugin for plugin and theme developers. It collects and displays useful debug information (errors, executed queries, globals, profiler).

John Blackbourn

Query Monitor is the developer tools panel for WordPress and WooCommerce. It enables debugging of database queries, PHP errors, hooks and actions, block editor blocks, enqueued scripts and stylesheets, HTTP API calls, and more.
It includes some advanced features such as debugging of Ajax calls, REST API calls, user capability checks, and full support for block themes and full site editing. It includes the ability to narrow down much of its output by plugin or theme, allowing you to quickly determine poorly performing plugins, themes, or functions.
Query Monitor focuses heavily on presenting its information in a useful manner, for example by showing aggregate database queries grouped by the plugins, themes, or functions that are responsible for them. It adds an admin toolbar menu showing an overview of the current page, with complete debugging information shown in panels once you select a menu item.
Query Monitor supports versions of WordPress up to three years old, and PHP version 7.4 or higher.
For complete information, please see <a href="https://querymonitor.com/" rel="nofollow ugc">the Query Monitor website</a>.
Here’s an overview of what’s shown for each page load:
<ul>
<li>Database queries, including notifications for slow, duplicate, or erroneous queries. Allows filtering by query type (<code>SELECT</code>, <code>UPDATE</code>, <code>DELETE</code>, etc), responsible component (plugin, theme, WordPress core), and calling function, and provides separate aggregate views for each.</li>
<li>The template filename, the complete template hierarchy, and names of all template parts that were loaded or not loaded (for block themes and classic themes).</li>
<li>PHP errors presented nicely along with their responsible component and call stack, and a visible warning in the admin toolbar.</li>
<li>Usage of “Doing it Wrong” or “Deprecated” functionality in the code on your site.</li>
<li>Blocks and associated properties within post content and within full site editing (FSE).</li>
<li>Matched rewrite rules, associated query strings, and query vars.</li>
<li>Enqueued scripts and stylesheets, along with their dependencies, dependents, and alerts for broken dependencies.</li>
<li>Language settings and loaded translation files (MO files and JSON files) for each text domain.</li>
<li>HTTP API requests, with response code, responsible component, and time taken, with alerts for failed or erroneous requests.</li>
<li>User capability checks, along with the result and any parameters passed to the capability check.</li>
<li>Environment information, including detailed information about PHP, the database, WordPress, and the web server.</li>
<li>The values of all WordPress conditional functions such as <code>is_single()</code>, <code>is_home()</code>, etc.</li>
<li>Transients that were updated.</li>
<li>Usage of <code>switch_to_blog()</code> and <code>restore_current_blog()</code> on Multisite installations.</li>
</ul>
In addition:
<ul>
<li>Whenever a redirect occurs, Query Monitor adds an HTTP header containing the call stack, so you can use your favourite HTTP inspector or browser developer tools to trace what triggered the redirect.</li>
<li>The response from any jQuery-initiated Ajax request on the page will contain various debugging information in its headers. PHP errors also get output to the browser’s developer console.</li>
<li>The response from an authenticated WordPress REST API request will contain an overview of performance information and PHP errors in its headers, as long as the authenticated user has permission to view Query Monitor’s output. An <a href="https://developer.wordpress.org/rest-api/using-the-rest-api/global-parameters/#_envelope" rel="nofollow ugc">an enveloped REST API request</a> will include even more debugging information in the <code>qm</code> property of the response.</li>
</ul>
By default, Query Monitor’s output is only shown to Administrators on single-site installations, and Super Admins on Multisite installations.
In addition to this, you can set an authentication cookie which allows you to view Query Monitor output when you’re not logged in (or if you’re logged in as a non-Administrator). See the Settings panel for details.
<h3>Other Plugins</h3>
I maintain several other plugins for developers. Check them out:
<ul>
<li><a href="https://wordpress.org/plugins/user-switching/" rel="ugc">User Switching</a> provides instant switching between user accounts in WordPress.</li>
<li><a href="https://wordpress.org/plugins/wp-crontrol/" rel="ugc">WP Crontrol</a> lets you view and control what’s happening in the WP-Cron system</li>
</ul>
<h3>Privacy Statement</h3>
Query Monitor is private by default and always will be. It does not persistently store any of the data that it collects. It does not send data to any third party, nor does it include any third party resources. <a href="https://querymonitor.com/privacy/" rel="nofollow ugc">Query Monitor’s full privacy statement can be found here</a>.
<h3>Accessibility Statement</h3>
Query Monitor aims to be fully accessible to all of its users. <a href="https://querymonitor.com/accessibility/" rel="nofollow ugc">Query Monitor’s full accessibility statement can be found here</a>.

Query Monitor – The developer tools panel for WordPress

If you are a WordPress Developer, this should be the first plugin that you should install.

Must have!


...it would be nice to have an option to deactivate the plugin on mobile devices. I noticed that it slows down the loading and time to be able to interact with the pages significantly. Sure, sometimes it would be useful to have it on mobile as well, and that could be added as an option.Otherwise it's a great, no the best plugin for that use case 😀👍 


Great, just one thing...


One of my must haves for plugin development from the beginning. 


Must have


Thank you for this ingenious plugin. It has already saved me hours of work debugging. It is also extremely helpful when optimising your own plugins. I have never had any problems with Query Monitor. I can only recommend it to every developer to use it as well.


Perfect for Debugging


The best debugging plugin for Wordpress dev. It gives a huge wealth of information about what’s going on under the hood. A must have for debugging queries and finding out simple bits of server information.


A must have for WP developer


Essential for any WP dev and debugging!


The Best WP Debug Plugin and Great Support


I use this plugin almost every day. Saved my time a hundred times.


Absolutely must have for WP development


Amazing tool. Beyond belief this is free.


Wordpress ecosystem at its best


At first impressions it looks like what the browser does.


Good plugin

Excellent plugin!


Query Monitor is an incredibly useful tool for developers and site administrators. It provides detailed insights into database queries, page load times, and resource consumption, making it a must-have for debugging and performance optimization.



I particularly appreciate how it highlights potential errors in the code and plugins during development, saving time and effort in troubleshooting. Its intuitive interface and in-depth reports make identifying issues straightforward.



Overall, it’s a fantastic plugin that I highly recommend to anyone building or maintaining WordPress websites.


Essential for Debugging and Performance Monitoring

best

Useful plugin


Sometimes you feel like your site is a black box and that is something that Query Monitor solves.  It provides insight into what is happening, shows errors, and gives you stats and timings for the page load process.  This helps in tracking down problems, in picking which plugins are the most performant, and when developing, to see what templates are being loaded.  Amazing plugin, for free.  Thank you John Blackbourn! 


An Amazing Tool

Query Monitor is the developer tools panel for WordPress and WooCommerce.

BlackBox Debug Bar vs Query Monitor – The developer tools panel for WordPress