Code requires PHP version 5 and some sections will require at least version 5.1.2. Increasingly, version 5.2.3 (released May 2007) is regarded as the oldest version that should be supported by advanced software systems. At the time of writing the code is believed to run on all released PHP versions up to 5.3.2.
Examples of SQL assume MySQL of at least version 4.1 although development will increasingly require version 5 which is now widely used by typical web hosting services.
The author's testing is all done using Linux systems running the Apache web server. Code will probably run on other platforms but has not been extensively tested on them.