Im not understanding how data goes (via ajax) from client side to serverside and

[That's what she said]

BACK again? Hey. SO i bought a book on AJAX and css in hopes of figuring out ajax. ive read thru the ajax chapter a few times...and still cant quite figure out how it works.

I think the part that I am most confused about is the .setRequestHeader() method.

here is the part of of the JS method (I now understand the XMLHttprequest part. this is the part after it)

pageRequest.onreadystatechange=function() {
var object = document.getElementById(objectID);
object.innerHTML = pageRequest.responseText;
}
if (dataToSend) {
var sendData = 'sendData=' + dataToSend;
pageRequest.open('POST',url,true);
pageRequest.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
pageRequest.send(sendData);
}
else {
pageRequest.open('GET',url,true)
pageRequest.send(null)
}

I am just not understanding

is it like...when the data is sent (using pagerequest.send(sendData) it sends the info to the php page and then it returns it aswell at the same time...? or...?

yeah, i just really need clarification on setRequestHeader...

Thanks!
I Think what I am trying to say (about the send/sent part) is:

Is it like the php code is included kind of? (like the php method include()... kind of at all? like the code just calls the php code (in a seperate file) as if it were in the same file??