\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.
\xampp\apache\logs\error.log
xampp
error_log
phpinfo()
I found it in: \xampp\php\logs\php_error_log
\xampp\php\logs\php_error_log
You can also open the xampp control panel and click on the Button "logs":
