Chapter 3. Making Decisions and Repeating Yourself

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 true or false. Section 3.1 explains how the interpreter decides which expressions and values are true and which are false.

Those true and false values are used by language constructs such as if( ) to decide whether to run certain statements in a program. The ins and outs of if( ) are detailed later in this chapter in Section 3.2. Use if( ) and similar constructs any time the outcome of a program depends on some changing conditions.

While true and false are the cornerstones of decision making, usually you want to ask more complicated questions, such as "is this user at least 21 years old?" or "does this user have a monthly subscription to the web site or enough money in their account to buy a daily pass?" Section 3.3, later in this chapter, explains PHP's comparison and logical operators. These help you express whatever kind of decision you need to make in a program, such as seeing whether numbers or strings are greater than or less than each other. You can also chain together decisions into a larger decision that depends on its pieces.

Decision making is also used in programs when you want to repeatedly execute certain statements — you need a way to indicate when the repetition should stop. Frequently, this is determined by a simple counter, such as "repeat 10 times." This is like asking the question "Have I repeated 10 times yet?" If so, then the program continues. If not, the action is repeated again. Determining when to stop can be more complicated, too — for example, "show another math question to a student until 6 questions have been answered correctly." Section 3.4, later in this chapter, introduces PHP's while( ) and for( ) constructs, with which you can implement these kinds of loops.