Chapter 2 covered the basics of how to represent data in PHP programs. A program full of data is only half complete, though. The other piece of the puzzle is using that data to control how the program runs, taking actions such as:
If an administrative user is logged in, print a special menu.
Print a different page header if it's after three o'clock.
Notify a user if new messages have been posted since she last logged in.
All of these actions have something in common: they make decisions about whether a certain logical condition involving data is true or false. In the first action, the logical condition is "Is an administrative user logged in?" If the condition is true (yes, an administrative user is logged in), then a special menu is printed. The same kind of thing happens in the next example. If the condition "is it after three o'clock?" is true, then a different page header is printed. Likewise, if "Have new messages been posted since the user last logged in?" is true, then the user is notified.
When making decisions, the PHP interpreter boils down an expression into
false. Section 3.1 explains how the interpreter decides
which expressions and values are
true and which are
false values are used by language constructs such as
if( ) to decide whether to run certain statements in a program. The ins and
if( ) are detailed later in this chapter in
Section 3.2. Use
if( ) and similar constructs any time ...