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WP_DEBUG_LOG is a companion to WP_DEBUG that causes all errors to also be saved to a debug.log log file
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).
Note that this allows you to write to log file using PHP’s built in error_log() function, which can be useful for instance when debugging Ajax events.
When set to true, the log is saved to debug.log in the content directory (usually wp-content/debug.log) within your site’s filesystem.
Alternatively, you can set it to a valid file path to have the file saved elsewhere.
define( 'WP_DEBUG_LOG', true ); -or- define( 'WP_DEBUG_LOG', '/tmp/wp-errors.log' );
WP_DEBUG_LOGto do anything,
WP_DEBUGmust be enabled (true).
You can turn off WP_DEBUG_DISPLAY independently.
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.
define( 'WP_DEBUG_DISPLAY', false );
For WP_DEBUG_DISPLAY to do anything, WP_DEBUG must be enabled (true).
You can control WP_DEBUG_LOG independently.