Even if you don't need a complete webserver in your pocket, it is nice for every webdesigner to browse those PHP files offline...
From the pages of http://www.apachefriends.org :
There is a basic versionXAMPP is an easy to install Apache distribution containing MySQL, PHP and Perl. XAMPP is really very easy to install and to use - just download, extract and start.
and a lite version, suitable for smaller devices
Portable installation (from the readme_en.txt):
Double-click xampp-control.exe to launch.Unpack the package to your usb stick or a partition of your choice. There it must be on the highest level like E:\ or W:\. It will build E:\xampp or W:\xampp or something like this. Please do not use the "setup_xampp.bat" for an USB stick installation!