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Loops PHP offers three types of loop constructs that all do the same thingrepeat a section of code a number of timesin slightly different ways. The while Loop The while keyword takes a condition in parentheses, and the code block that follows is repeated while that condition is true. If the condition is false initially, the code block will not be repeated at all. Infinite Loops The repeating code must perform some action that affects the condition in such a way that the loop condition will eventually no longer be met; otherwise, the loop will repeat forever. The following example uses a while loop to display the square numbers from 1 to 10: $count = 1; while ($count <= 10) { $square = $count * $count; echo "$count squared is $square <br>"; $count++; } The counter variable $count is initialized with a value of 1. The while loop calculates the square of that number and displays it, then adds one to the value of $count. The ++ operator adds one to the value of the variable that precedes it. The loop repeats while the condition $count <= 10 is true, so the first 10 numbers and their squares are displayed in turn, and then the loop ends. The do Loop The do loop is very similar to the while loop except that the condition comes after the block of repeating code. Because of this variation, the loop code is always executed at least onceeven if the condition is initially false. The following do loop is equivalent to the previous example, displaying the numbers from 1 to 10, with their squares: $count = 1; do { $square = $count * $count; echo "$count squared is $square <br>"; $count++; } while ($count <= 10); The for Loop The for loop provides a compact way to create a loop. The following example performs the same loop as the previous two examples: for ($count = 1; $count <= 10; $count++) { $square = $count * $count; echo "$count squared is $square <br>"; } As you can see, using for allows you to use much less code to do the same thing as with while and do. A for statement has three parts, separated by semicolons:  The first part is an expression that is evaluated once when the loop begins. In the preceding example, you initialized the value of $count.  The second part is the condition. While the condition is true, the loop continues repeating. As with a while loop, if the condition is false to start with, the following code block is not executed at all.  The third part is an expression that is evaluated once at the end of each pass of the loop. In the previous example, $count is incremented after each line of the output is displayed. Nesting Conditions and Loops So far you have only seen simple examples of conditions and loops. However, you can nest these constructs within each other to create some quite complex rules to control the flow of a script. Remember to Indent The more complex the flow control in your script is, the more important it becomes to indent your code to make it clear which blocks of code correspond to which constructs. Breaking Out of a Loop You have already learned about using the keyword break in a switch statement. You can also use break in a loop construct to tell PHP to immediately exit the loop and continue with the rest of the script. The continue keyword is used to end the current pass of a loop. However, unlike with break, the script jumps back to the top of the same loop and continues execution until the loop condition fails. Summary In this lesson you have learned how to vary the flow of your PHP script by using conditional statements and loops. In the next lesson you will see how to create reusable functions from blocks of PHP code. Lesson 4. Functions In this lesson you will learn how frequently used sections of code can be turned into reusable functions. Using Functions A function is used to make a task that might consist of many lines of code into a routine that can be called using a single instruction. PHP contains many functions that perform a wide range of useful tasks. Some are built in to the PHP language; others are more specialized and are available only if certain extensions are activated when PHP is installed. The online PHP manual (www.php.net) is an invaluable reference. As well as documentation for every function in the language, the manual pages are also annotated with user-submitted tips and examples, and you can even submit your own comments if you want. Online Reference To quickly pull up the PHP manual page for any function, use this shortcut: www.php.net/function_name. You have already used the date function to generate a string that contains a formatted version of the current date. Let's take a closer look at how that example from Lesson 1, "Getting to Know PHP," works. The example looked like this: echo date('j F Y'); The online PHP manual gives the prototype for date as follows: string date (string format [, int timestamp]) This means that date takes a string argument called format and, optionally, the integer timestamp. It returns a string value. This example sends j F Y to the function as the format argument, but timestamp is omitted. The echo command displays the string that is returned. Prototypes Every function has a prototype that defines how many arguments it takes, the arguments' data types, and what value is returned. Optional arguments are shown in square brackets ([]). Defining Functions In addition to the built-in functions, PHP allows you to define your own. There are advantages to using your own function. Not only do you have to type less when the same piece of code has to be executed several times but a custom-defined function also makes your script easier to maintain. If you want to change the way a task is performed, you only need to update the program code oncein the function definitionrather than fix it every place it appears in your script. Modular Code Grouping tasks into functions is the first step toward modularizing your codesomething that is especially important to keep your scripts manageable as they grow in size and become more complex. The following is a simple example that shows how a function is defined and used in PHP: function add_tax($amount) { $total = $amount * 1.09; return $total; } $price = 16.00; echo "Price before tax: $price <br>"; echo "Price after tax: "; echo add_tax($price); The function keyword defines a function called add_tax that will execute the code block that follows. The code that makes up a function is always contained in braces. Putting $amount in parentheses after the function name stipulates that add_tax takes a single argument that will be stored in a variable called $amount inside the function. The first line of the function code is a simple calculation that multiplies $amount by 1.09which is equivalent to adding 9% to that valueand assigns the result to $total. The return keyword is followed by the value that is to be returned when the function is called from within the script. Running this example produces the following output: Price before tax: 16 Price after tax: 17.44 This is an example of a function that you might use in many places in a web page; for instance, on a page that lists all the products available in an online store, you would call this function once for each item that is displayed to show the after-tax price. If the rate of tax changes, you only need to change the formula in add_tax to alter every price displayed on that page. . code to do the same thing as with while and do. A for statement has three parts, separated by semicolons:  The first part is an expression that is evaluated once when the loop begins. In the. the loop begins. In the preceding example, you initialized the value of $count.  The second part is the condition. While the condition is true, the loop continues repeating. As with a while. condition is false to start with, the following code block is not executed at all.  The third part is an expression that is evaluated once at the end of each pass of the loop. In the previous