AJAX and PHP

Server-Side Techniques with PHP and MySQL
function error_handler($errNo, $errStr, $errFile, $errLine)
{
// clear any output that has already been generated
if(ob_get_length()) ob_clean();
// output the error message
$error_message = 'ERRNO: ' . $errNo . chr(10) .
'TEXT: ' . $errStr . chr(10) .
'LOCATION: ' . $errFile .
', line ' . $errLine;
echo $error_message;
// prevent processing any more PHP scripts
exit;
}
?>
6. Load http://localhost/ajax/foundations/smartproxyping/
smartproxyping.html. The output should look like the one in Figure 3.15.
What Just Happened?
Our client, in this example, knows how to check from time to time if the server is available. The
random number generator service provides the page http://www.random.org/cgi-bin/checkbuf
—which you can use to check its buffer level.
The JavaScript code in smartproxyping.js starts by defining a number of global variables that
you use to control the program's behavior:
// holds the remote server address and parameters
var serverAddress = "smartproxyping.php";
var getNumberParams = "action=GetNumber" + // get a new random number
"&min=1" + // the min number to generate
"&max=100"; // the max number to generate
var checkAvailabilityParams = "action=CheckAvailability";
// variables used to check for server availability
var requestsCounter = 0; // counts how many numbers have been retrieved
var checkInterval = 10; // counts interval for checking server availability
var updateInterval = 1; // how many seconds to wait to get a new number
var updateIntervalIfServerBusy = 10; // seconds to wait when server busy
var minServerBufferLevel = 50; // what buffer level is considered acceptable
These variables contain the data required to make server requests. getNumberParams contains the
query string parameters needed to request a new random number, and checkAvailabilityParams
contains the parameters used to check the server's buffer level. The other variables are used to
control the intervals for making the asynchronous requests.
A novelty in this exercise compared to the previous ones is that you have two functions that
handle server responses—handleCheckingAvailability and handleGettingNumber. The roots of
this happen to be in the process() function, which assigns one of these callback functions
depending on the server action it requests.
In this program, process() is not called only once as in other exercises; instead, it is called
multiple times, and each time it must decide what action to make—should it ask for a new random
number, or should it check the server's buffer level? The requestsCounter variable, which keeps
a track of how many times we have retrieved a new random number since the last buffer check,
helps us make a decision:
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