Online documentation

XAMPP combines many different sofware packages into one package. Here's a list of standard and reference documentation of the most important packages.

Apache 2 documentation
PHP referenz documentation
Perl documentation
MySQL documentation
ADODB
eAccelerator
FPDF Class documentation

And a small list of tutorials and the Apache Friends documentation page:

Apache Friends documentation
PHP Tutorial by David Gowans
HTML - An Interactive Tutorial For Beginners by Dave Kristula
Perl Tutorial by Nik Silver

Good luck and have fun! :)