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.

The rest of this book contains a quick-reference for the core and client-side JavaScript APIs. It documents the complete core JavaScript API, covers the legacy (Level 0) DOM API, and presents a simplified view of the W3C Level 2 DOM API. Portions of that API not relevant to JavaScript programmers working with HTML documents have been omitted. The upper-right corner of the title block for each reference entry contains information that states whether a feature is part of the core or client-side API, and further indicates which version of JavaScript, which browsers, or which version of the DOM introduced the feature.

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:

Log files track the activity on a computer so administrators can see who has used the computer, what they did, how long they stayed connected, and where they came from. Since a log file can also record the activity of an unauthorized intruder, much like a surveillance camera can record a burglar breaking into a store, hackers look for the log file as soon as they get access into the computer. Script kiddies often delete the log files to prevent the administrator from seeing exactly what they did on the computer. Unfortunately, deleting the log file reveals the presence of an intruder as blatantly as using a stick of dynamite to get rid of a surveillance camera. The moment an administrator notices that someone has deleted the log file, he or she immediately knows that a hacker must be on the system.