javascript ajax php调用返回3个以上的字符

i have this php file (stripped down to the essential part)

getBoundaries.php

<?php
        $t = "";
        //$t = "t";

        Header("Content-type: text/plain; charset=utf-8");
        echo ($t);
 ?>

and this javascript ajax call:

function anyname(){
    var xhttp;
    var query = "tid=1&pid=1";
    xhttp = new XMLHttpRequest();
    xhttp.onreadystatechange = function() {
        if (xhttp.readyState == 4 && xhttp.status == 200) {
            var temp = xhttp.responseText;
            if (temp == ""){
                console.log("ein leerer string");
            }
            else{
                for (i = 0; i < temp.length; i++){
                    console.log(temp.charCodeAt(i));
                }
            }
        }
    }
    xhttp.open("POST", "getBoundaries.php", true);
    xhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
    xhttp.send(query);
}

The output of console.log with $t="" were 32, 10, 32.

The output of console.log with $t="t" were 116, 32, 10, 32.

So, my question is: WHY isn't an empty string returned as responseText? Where are these additional 3 characters added? My assumption: the echo php-command adds those three chars.

Any suggestions?