WordPress debugging

If WP_DEBUG_LOG is set to true, the error_log-INI setting is set:

ini_set( 'error_log', WP_CONTENT_DIR . '/debug.log' );

To write to that file, you can use the error_log-function:

error_log("This message is written to the log file");

This function is not specific to WordPress and can be used in any PHP script.

Here's a simple function you can use; it will only log a message if WP_DEBUG is enabled:

function log_me($message) {
    if ( WP_DEBUG === true ) {
        if ( is_array($message) || is_object($message) ) {
            error_log( print_r($message, true) );
        } else {
            error_log( $message );
        }
    }
}

You can call the log_me() function like this in your theme template(s):

log_me( 'This is a message for debugging purposes' );

Which will appear in your /wp-content/debug.log as the following line:

[13-Apr-2013 20:59:17 UTC] This is a message for debugging purposes