AJAX postdata working on localhost but not on (Apache) server - ajax

Question

I identified an issue with postdata not being sent through AJAX on my server. To debug it, I wrote the following fairly minimalistic piece of javascript to test a simple AJAX call :
function my_custom_ajax(target_page, target_element , postdata_contents) {
// Sending the XMLHttpRequest as postdata
var xhr = new XMLHttpRequest();
xhr.open("POST", target_page, true);
xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded;");
xhr.setRequestHeader("Content-length", postdata_contents.length);
xhr.setRequestHeader("Connection", "close");
xhr.send(postdata_contents);
// Waiting for the request to return
xhr.onreadystatechange = return_data;
// If all went well, we update the target element
function return_data()
{
if(xhr.readyState === 4)
{
if(xhr.status === 200)
{
// We update the target element
document.getElementById(target_element).innerHTML = xhr.responseText;
}
// Throw error in case of 404 or such
else
document.getElementById(target_element).innerHTML = "XHR can't be loaded";
}
// Throw error in case request got interrupted or didn't work out
else
document.getElementById(target_element).innerHTML = "XHR error";
}
}
It is called with the following HTML :
<div onClick="my_custom_ajax('test_page.php?xhr','my_id','postdata_test');">
Click me
</div>
<div id="my_id">
xhr response will appear here
</div>
And calls a PHP page which contains only this :
exit(var_dump($_POST));
When running this piece of code in my Apache localhost or another Apache server I own, it does pass whatever is in postdata_contents as postdata. The exit(var_dump($_POST)); does show that it works properly, and does print the value of the postdata I passed to it.
However, when running this same piece of code on the Apache server where it does not work, all I get is « array(0) { } », as in, no postdata is passed according to the PHP file.
Here is Firefox's dev tool view of the request details (in french, sorry, but should be obvious what is what) :
The debug tool shows that the postdata contents are properly being sent :
However, the returned content show that the postdata was somehow not passed :
On my localhost and my other Apache server, everything is exactly the same until the very last step, where the postdata is properly passed (the var_dump message is styled but you can easily see the gist of it : postdata_test is part of $_POST) :
After hours of fiddling with the configuration of this Apache server and trying all of the debug methods and breakpoints I could think up, my nerves are too worked up to continue thinking about this rationally for now. As I have no option of using another server or just copypasting my local Apache configuration file on the new server, I defer this question to you all, hoping that somebody can figure it out or once encountered something similar.
Thanks in advance,
Eric B.

Answer 1

Solved it myself by accident, I had mod_dumpio activated on the server and it started working once I turned it off.
I do not know what mod_dumpio was doing to deny XHR POST but not generic HTTP POST, but at least that's solved.
Hope this will help someone else some day.
(on a sidenote, I realize the postdata query in my example was malformed, should have been « postdata_test= » instead of « postdata_test », so add that equal sign if you are stuck in my situation and want to run the same tests I did)