只用javascript调用Ajax（没有jquery）

I'm trying to do an ajax call with POST method, but the PHP only return an empty array. What am i doing wrong?

JAVASCRIPT

// ajax call

function makeRequest(){

    var http_request = false;
    // example data
    var fileObjectInfo = 'bla';
    var url = 'archivo.php';

    if (window.XMLHttpRequest){
        http_request = new XMLHttpRequest();
        if (http_request.overrideMimeType) {
            http_request.overrideMimeType('text/xml');
        }
    }else if(window.ActiveXObject){
        try{
            http_request = new ActiveXObject("Msxml2.XMLHTTP");
        }catch(e){
            try{
                http_request = new ActiveXObject("Microsoft.XMLHTTP");
            }catch(e){}
        }
    }

    if (!http_request) {
        console.log('Falla :( No es posible crear una instancia XMLHTTP');
        return false;
    }


    http_request.onreadystatechange = alertContents;
    http_request.open('POST', url, true);
    http_request.send(fileObjectInfo);


    function alertContents(){

        if (http_request.readyState == 4) {
            if (http_request.status == 200) {

                console.log(http_request.responseText);

            } else {
                console.log('Hubo problemas con la petición.');
            }
        }
    }
}

My php only do: <?php print_r($_POST); ?> (i tried with request too), but always return an emty array. (so the call works but javascrip doesn't send the information, no?)

I get examples code from internet but never works the ajax call with POST method and i don't know why.

thank you in advance for all the help they can lend.

If You request post method you must be pass header data with request like

http_request.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
http_request.setRequestHeader("Content-length", fileObjectInfo.length);
http_request.setRequestHeader("Connection", "close");

put this data before http_request.send(fileObjectInfo);

I hope this will help you

To send a POST request you need a form data object containing key value pairs for PHP to interpret post values

var fileObjectInfo = new FormData();
fileObjectInfo.append("key", "value");
fileObjectInfo.append("name", "prince");

// ajax code
http_request.send(fileObjectInfo);

In PHP then do this

echo $_POST["key"];
echo $_POST["name"];

Be sure to check if the post params exist using isset()