Debugging PHP code is part of any project, but WordPress comes with specific debug systems designed to simplify the process as well as standardize code across the core, plugins and themes. This page describes the various debugging tools in WordPress and how to be more productive in your coding as well as increasing the overall quality and interoperativity of your code.
NOTE: While it is not mandatory to account for WP_DEBUG in plugins and themes it is highly recommended that plugin and theme developers use WP_DEBUG mode while working on code they plan to release publicly. If your plugin or theme is not compatible then the errors, notices and warnings it throws will make it impossible for other developers to use your plugin/theme while they have WP_DEBUG enabled and your theme will not be eligible for promotion via the official WordPress tools.
WP_DEBUG is a PHP constant (a permanent global variable) that can be used to trigger the “debug” mode throughout WordPress. It is assumed to be false by default and is usually set to true in the wp-config.php file on development copies of WordPress.
Note: The true and false values in the example are not surrounded by apostrophes (‘) because they are boolean (true/false) values. If you set constants to ‘false’, they will be interpreted as true because the quotes make it a string rather than a boolean.
It is not recommended to use WP_DEBUG or the other debug tools on live sites; they are meant for local testing and staging installs.
Enabling WP_DEBUG will cause all PHP errors, notices and warnings to be displayed. This is likely to modify the default behavior of PHP which only displays fatal errors and/or shows a white screen of death when errors are reached.
Showing all PHP notices and warnings often results in error messages for things that don’t seem broken, but do not follow proper data validation conventions inside PHP. These warnings are easy to fix once the relevant code has been identified, and the resulting code is almost always more bug-resistant and easier to maintain.
Enabling WP_DEBUG will also cause notices about deprecated functions and arguments within WordPress that are being used on your site. These are functions or function arguments that have not been removed from the core code yet but are slated for deletion in the near future. Deprecation notices often indicate the new function that should be used instead.
WP_DEBUG_LOG is a companion to WP_DEBUG that causes all errors to also be saved to a debug.log log file inside the /wp-content/ directory. This is useful if you want to review all notices later or need to view notices generated off-screen (e.g. during an AJAX request or wp-cron run).
WP_DEBUG_DISPLAY is another companion to WP_DEBUG that controls whether debug messages are shown inside the HTML of pages or not. The default is ‘true’ which shows errors and warnings as they are generated. Setting this to false will hide all errors. This should be used in conjunction with WP_DEBUG_LOG so that errors can be reviewed later.
The SAVEQUERIES definition saves the database queries to an array and that array can be displayed to help analyze those queries. The constant defined as true causes each query to be saved, how long that query took to execute, and what function called it.
The array is stored in the global $wpdb->queries.
NOTE: This will have a performance impact on your site, so make sure to turn this off when you aren’t debugging.
The following code, inserted in your wp-config.php file, will log all errors, notices, and warnings to a file called debug.log in the wp-content directory. It will also hide the errors so they do not interrupt page generation.
// Enable WP_DEBUG mode
// Enable Debug logging to the /wp-content/debug.log file
// Disable display of errors and warnings
// Use dev versions of core JS and CSS files (only needed if you are modifying these core files)
There are many well-written plugins that handle debugging in WordPress and show more information about the internals, either for a specific component or in general. Some examples of such plugins are Debug Bar with Debug Bar Console, Log Deprecated Notices and Total Security.