After upgrading XAMPP, I started getting "Call to undefined function" errors on web pages which previously worked.
The error was NOT related to php.ini, modules, or the like. It involved the calling of a function included in a require/include file. The file was successfully accessed, but the function was not recognized.
After an enormous amount of time spent isolating this error (!!), I discovered that the include file did not have "php" after the "<?" at the beginning of the file. After changing "<?" to "<?php", the function worked.
I hope this helps someone!