Where does PHP's error log reside in XAMPP?

\xampp\apache\logs\error.log, where xampp is your installation folder. If you haven't changed the error_log setting in PHP (check with phpinfo()), it will be logged to the Apache log.

I found it in: \xampp\php\logs\php_error_log

You can also open the xampp control panel and click on the Button "logs":