AJAX and PHP

Server-Side Techniques with PHP and MySQL
What Just Happened?
Most of the principles implemented in the application were covered earlier in the book, so we will
quickly analyze what's new here, starting from the client-side code. The novelty in index.html
consists in using the element to offer a minimal support for browsers that don't
support JavaScript, or for ones whose JavaScript support has been disabled:
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This example requires a JavaScript-enabled browser!
Browsers that have JavaScript enabled will ignore everything between and
, while the others will parse and display that HTML code.
The client-side JavaScript file, friendly.js has a few surprises of its own:
• We grouped common functionality that handles displaying user messages into the
display and displayError functions. Both receive as parameter the message to be
displayed, but displayError displays the message only if debugMode is true (this
variable is defined at the beginning of the file).
• displayError is called in the catch blocks after an exception has been thrown
somewhere, and it uses setTimeout to restart the sequence that makes server
requests. You can set how much time the script should wait before attempting
a new server request when an error happens by modifying the value of the
errorRetryInterval variable.
• You can change how often the news message should be displayed by changing the
updateInterval variable.
• In getNews(), we have a simplistic mechanism that checks whether the text received
from the server was a server-side error instead of the message we are waiting for.
This mechanism verifies if the response contains "ERRNO" (which is generated by
our server-side custom error handler), or "error" (which is generated automatically
by PHP in the case of fatal errors or parse errors), or if the response is empty (if the
displayErrors option is set to Off in php.ini, no error text is generated). In any
of these cases, we throw an error manually, which is then received by our
error-handling mechanism that informs the users that an error has happened.
At the server side, everything starts in friendly.php, which is called from the client. The most
important part of friendly.php is the one where it creates a new instance of the Friendly class
(defined in friendly.class.php), and calls its getNews method:
// read the action parameter
$action = $_GET['action'];
// get news
if ($action == 'GetNews')
{
// create new instance of the Friendly class
$friendly = new Friendly();
// use Friendly functionality to retrieve the news message
$news = $friendly->getNews();
// echo the message to be read by the client
echo $news;
}
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