By default, WordPress debugging is turned off, so to enable it, open wp-config.php (tip: make a backup copy of this file that you can revert to later if needed) in the root of your WordPress installation and look for this line:
define('WP_DEBUG', false);
Replace that line with the following:
// Turns WordPress debugging on
define('WP_DEBUG', true);
// Tells WordPress to log everything to the /wp-content/debug.log file
define('WP_DEBUG_LOG', true);
// Doesn't force the PHP 'display_errors' variable to be on
define('WP_DEBUG_DISPLAY', false);
// Hides errors from being displayed on-screen
@ini_set('display_errors', 0);
With those lines added to your wp-config.php file, debugging is fully enabled. Here’s an example of a notice that got logged to /wp-content/debug.log for using a deprecated function:
[15-Feb-2011 20:09:14] PHP Notice: get_usermeta is deprecated since version 3.0! Use get_user_meta() instead. in C:\Code\Plugins\wordpress\wp-includes\functions.php on line 3237
With debugging enabled, keep a close eye on /wp-content/debug.log as you develop your plugin. Doing so will save you, your users, and other plugin developers a lot of headaches.