WordPress Custom debug.log function

WordPress Custom debug.php

if (!function_exists('write_log')) {
    function write_log ( $log )  {
        if ( true === WP_DEBUG ) {
            if ( is_array( $log ) || is_object( $log ) ) {
                error_log( print_r( $log, true ) );
            } else {
                error_log( $log );
            }
        }
    }
}

//calling function
write_log('THIS IS THE START OF MY CUSTOM DEBUG');

write_log($whatever_you_want_to_log);

Source:http://www.stumiller.me/sending-output-to-the-wordpress-debug-log/