When we think of Trojans, we normally think of the huge wooden horse that the Greeks gifted the city of Troy, in which they had secretly hidden a small number of their warriors. When the Trojans took the horse into the city the soldiers emerged and opened the gates to the cities enemies. Computer trojans are based on the same idea; what can appear to be useful program or harmless site can be hiding a small Trojan program that once it has infected your computer opens a secret backdoor that will allow an intruder to remotely gain access to your computer. Trojans are not viruses in that they are not designed to reproduce themselves. Instead they rely on being small and deceptive to propagate themselves across the internet, usually attached to some other file.

The genesis of the computer revolution was in a machine. The genesis of our programming languages thus tends to look like that machine.But computers are not so much machines as they are mind amplification tools (“bicycles for the mind,” as Steve Jobs is fond of saying) and a different kind of expressive medium. As a result, the tools are beginning to look less like machines and more like parts of our minds, and also like other expressive mediums such as writing, painting, sculpture, animation, and filmmaking. Object-oriented programming is part of this movement toward using the computer as an expressive medium.

You will create two main types of variables in your PHP code: scalar and array. Scalar variables contain only one value at a time, and arrays contain a list of values, or even another array.The example at the beginning of this chapter created a scalar variable, and the code in this book deals primarily with scalar variables. You can find information on arrays in Appendix B, "Basic PHP Language Reference." When you assign a value to a variable, you usually assign a value of one of the following types:

Just because Linux is distributed for free, it doesn’t mean the software is not copyrighted. Linux has been registered as copyrighted under the GNU General Public License (GPL), which is known in the programming community as a copyleft instead of copyright because of its nature. The GPL allows you to redistribute the Linux software, along with the complete source code, to anyone who wants it. However, the original owner of the components retains the copyrights to the software.