AJAX and PHP

Server-Side Techniques with PHP and MySQL
The error-handling scheme presented is indeed quite simplistic, and it is only appropriate
while writing and debugging your code. In a production solution, you need to show your
end user a friendly message without any technical details. If you want to package the
error details as an XML document to be read on the client, keep in mind that parse and
fatal errors will not be processed by your function, and will behave as set up in PHP's
configuration file (php.ini).
This case also presents the scenario where the user can attempt to make several server requests at
the same time (you can do this by clicking the Send button multiple times quickly enough). If you
try to make a request on a busy XMLHttpRequest object, its open method generates an exception.
The code is well protected with try/catch constructs, but the error message doesn't look very
user-friendly as shown in Figure 3.6.
Figure 3.6: Request on a Busy XMLHttpRequest
This message might be just what you need, but in certain circumstances you may prefer to react
differently to this kind of error than with other kinds of errors. For example, in a production
scenario, you may prefer to display a note on the page, or display a friendly "please try again
later" message, by modifying the process() function as shown in the following code snippet:
// read a file from the server
function process()
{
// only continue if xmlHttp isn't void
if (!xmlHttp) return;
// don't try to make server requests if the XMLHttpObject is busy
if !(xmlHttp.readyState == 0 || xmlHttp.readyState == 4)
alert("Can't connect to server, please try again later.");
else
{
// try to connect to the server
try
{
// get the two values entered by the user
var firstNumber = document.getElementById("firstNumber").value;
var secondNumber = document.getElementById("secondNumber").value;
// create the params string
var params = "firstNumber=" + firstNumber +
"&secondNumber=" + secondNumber;
// initiate the asynchronous HTTP request
xmlHttp.open("GET", "morephp.php?" + params, true);
xmlHttp.onreadystatechange = handleRequestStateChange;
xmlHttp.send(null);
}
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