If your server runs php as an apache module, you'll need to use your .htaccess file. If your server runs php as a CGI module, you can use a php.ini file to update the settings.
That's a bit confusing I know if you're already not familiar with updating php settings.
I found that this page may help:
Configure Error Log
Because wp-config.php is loaded for every page view not loaded from a cache file, it is an excellent location to set php ini settings that control your php installation. This is useful if you don't have access to a php.ini file, or if you just want to change some settings on the fly.
Here is an example that turns php error_logging on and logs them to a specific file. If WP_DEBUG is defined to true, the errors will also be saved to this file. Just place this above any require_once or include commands.
/* That's all, stop editing! Happy blogging. */