How can I use error_log() with WordPress?

Update 2019

You can simply enable debugging by adding the following code to your wp-config.php:

define( 'WP_DEBUG', true );
define( 'WP_DEBUG_LOG', true );

There will be a debug.log file generated in your wp-content folder.

If you want to log the errors without printing them in the frontend, add the following line:

define( 'WP_DEBUG_DISPLAY', false );

This is really useful in production environments, since the page visitor will not be able to see your logs.

Now you can just write to your log by using the error_log function:

error_log( 'Hello World!' );

Or pretty print your output by making use of the print_r method:

error_log( print_r( 'Hello World!', true ) );

Pro Tip: If you're using the bash you can observe the log with tail -f wp-content/debug.log

According to the Codex, WP_DEBUG_DISPLAY should be set to true by default, but it would seem that was not the case.

Adding define('WP_DEBUG_DISPLAY', true); to wp_config.php fixed the error logging.

Setting WP_DEBUG_DISPLAY to false removed the errors from the browser but allowed them to be output in the log.

It would seem that Wordpress requires define('WP_DEBUG_DISPLAY'); to output errors to the log whether you set it to be true or false.

I have the same problem. For me commenting out this line helped.

define( 'WP_DEBUG_LOG', true );