Giải pháp thiết kế web động với PHP - p 9

HOW TO WRITE PHP SCRIPTSThe main points to note about switch are as follows:• • • • •The expression following the case keyword must be a number or a string. You can t use comparison operators with case. So case 100: isn t allowed. Each block of statements should normally end with break, unless you specifically want to continue executing code within the switch statement. You can group several instances of the case keyword together to apply the same block of code to them. If no match is made, any statements following the default keyword are executed. If no...
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Giải pháp thiết kế web động với PHP - p 9 HOW TO WRITE PHP SCRIPTS The main points to note about switch are as follows: • The expression following the case keyword must be a number or a string. • You can t use comparison operators with case. So case > 100: isn t allowed. • Each block of statements should normally end with break, unless you specifically want to continue executing code within the switch statement. • You can group several instances of the case keyword together to apply the same block of code to them. • If no match is made, any statements following the default keyword are executed. If no default has been set, the switch statement exits silently and continues with the next block of code.Using the ternary operator The ternary operator (?:) is a shorthand method of representing a simple conditional statement. Its name comes from the fact that it normally uses three operands. The basic syntax looks like this: condition ? value if true : value if false ; Here is an example of it in use: $age = 17; $fareType = $age > 16 ? adult : child; The second line tests the value of $age. If it s greater than 16, $fareType is set to adult, otherwise $fareType is set to child. The equivalent code using if . . . else looks like this: if ($age > 16) { $fareType = adult; } else { $fareType = child; } The if . . . else version is easier to read, but the conditional operator is more compact. Most beginners hate this shorthand, but once you get to know it, you ll realize how convenient it can be. In PHP 5.3 and later, you can leave out the value between the question mark and the colon. This has the effect of assigning the value of the condition to the variable if the condition is true. In the preceding example, leaving out the value between the question mark and the colon results in $fareType being true: $age = 17; $fareType = $age > 16 ?: child; // $fareType is true In this case, the result is almost certainly not what you want. This shorthand is useful when the condition is a value that PHP treats as implicitly true, such as an array with at least one element. Omitting the value between the question mark and the colon is a specialized use of the ternary operator and is not used in the scripts in this book. It is mentioned here only to alert you to its meaning if you come across it elsewhere. 61 CHAPTER 3 Creating loops A loop is a section of code that is repeated over and over again until a certain condition is met. Loops are often controlled by setting a variable to count the number of iterations. By increasing the variable by one each time, the loop comes to a halt when the variable gets to a preset number. The other way loops are controlled is by running through each item of an array. When there are no more items to process, the loop stops. Loops frequently contain conditional statements, so although they re very simple in structure, they can be used to create code that processes data in often sophisticated ways. Loops using while and do . . . while The simplest type of loop is called a while loop. Its basic structure looks like this: while ( condition is true ) { do something } The following code displays every number from 1 through 100 in a browser (you can test it in while.php inDownload from Wow! eBook the files for this chapter). It begins by setting a variable ($i) to 1 and then using the variable as a counter to control the loop, as well as display the current number onscreen. $i = 1; // set counter while ($i HOW TO WRITE PHP SCRIPTS The danger with while and do . . . while loops is forgetting to set ...