AJAX and PHP

Chapter 3
On the server side, all the interesting things happen in friendly.class.php, which is called from
friendly.php to do the interesting part of the work. In friendly.class.php you can find the
Friendly class, which has the following four members:
• $mMysqli: A private field that stores an open database connection during the life of
the object.
• __construct(): The class constructor initializes $mMysqli by opening a database
connection. Because the constructor is executed automatically when an instance of
the class is created, you can safely assume to have the connection available in all
methods of the class.
• __destruct(): The class destructor closes the database connection. The destructor is
executed automatically when the class instance is destroyed.
• getRandomNumber(): This is a private helper method that returns a random number.
Private methods can't be called from programs that create instances of the class, and
are meant to provide internal functionality only. The code in getRandomNumber is
familiar from the previous exercises, as it calls the external random.org server to
retrieve new random numbers. The usleep PHP function is used to artificially add a
quarter of a second delay, so that you can admire the "Receiving new message from
server…" message on the client for a little longer.
• getNews(): This is a public method that an external program can access to get a
new "news" message. The method gets two random user names from the database,
uses the getRandomNumber method to retrieve a random number, and composes a
message such as "User x works with user y at project #z". (Yes that's not very
imaginative but we couldn't think of anything more interesting—sorry!) Note the
$this special object that is used to access $mMysqli and getRandomNumber().
Class members can only be accessed using an instance of the class and in PHP
$this refers to the current class instance.
Summary
Hopefully, you have enjoyed the little examples of this chapter, because many more will follow!
This chapter walked you through the technologies that live at the server side of a typical AJAX
application. We have done a few exercises that involved simple server functionality, and PHP did
a wonderful job at delivering that functionality. You have also learned the basics of working with
databases, and simple database operations with the first table created in this book.
In the following chapters, you'll meet even more interesting examples that use more advanced
code to implement their functionality. In Chapter 4, you'll build an AJAX-enabled form validation
page, which is safe to work even if the client doesn't support JavaScript and AJAX.
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