AJAX and PHP

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Client-Side Techniques with Smarter JavaScript
The third parameter of open, called async, specifies whether the request should be handled
asynchronously; true means that script processing carries on after the send() method returns
without waiting for a response; false means that the script waits for a response before
continuing processing, freezing the web page functionality. To enable asynchronous processing,
you will seed to set async to true, and handle the onreadystatechange event to process the
response from the server.
When using GET to pass parameters, you send the parameters using the URL's query string, as
in http://localhost/ajax/test.php?param1=x&param2=y. This server request passes two
parameters—a parameter called param1 with the value x, and a parameter called param2 with the
value y.
// call the server page to execute the server side operation
xmlHttp.open("GET", "http://localhost/ajax/test.php?param1=x&param2=y", true);
xmlHttp.onreadystatechange = handleRequestStateChange;
xmlHttp.send(null);
When using POST, you send the query string as a parameter of the send method, instead of joining
it on to the base URL, like this:
// call the server page to execute the server side operation
xmlHttp.open("POST", "http://localhost/ajax/test.php", true);
xmlHttp.onreadystatechange = handleRequestStateChange;
xmlHttp.send("param1=x&param2=y");
The two code samples should have the same effects. In practice, using GET can help with
debugging because you can simulate GET requests with a web browser, so you can easily see with
your own eyes what your server script generates. The POST method is required when sending data
larger than 512 bytes, which cannot be handled by GET.
In our examples, we will place the code that makes the HTTP request inside a function called
process() in the JavaScript file. The minimal implementation, which is quite fragile and doesn't
implement any error-handling techniques, looks like this:
function process()
{
// call the server page to execute the server side operation
xmlHttp.open("GET", "server_script.php", true);
xmlHttp.onreadystatechange = handleRequestStateChange;
xmlHttp.send(null);
}
This method has the following potential problems:
• process() may be executed even if xmlHttp doesn't contain a valid
XMLHttpRequest instance. This may happen if, for example, the user's browser
doesn't support XMLHttpRequest. This would cause an unhandled exception to
happen, so our other efforts to handle errors don't help very much if we aren't
consistent and do something about the process function as well.
• process() isn't protected against other kinds of errors that could happen. For
example, as you will see later in this chapter, some browsers will generate a security
exception if they don't like the server you want to access with the XMLHttpRequest
object (more on security in Chapter 3).
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