Why a bunch of HTML tags appear in the value of the $_ post superglobal variable

submit data to the server with a post request through Ajax, and output the value of the $_ post superglobal variable in PHP. The output is shown in the following figure

:

//demo3.html

<!DOCTYPE html>

<html lang="en">

<head>

<meta charset="UTF-8">

<meta name="viewport" content="width=device-width, initial-scale=1.0">

<meta http-equiv="X-UA-Compatible" content="ie=edge">

<title>Document</title>

</head>

<body>

    <form action="Demo3.php" method="POST">
    
    :<input type="text" name="username" id="un"><br/>
    
    :<input type="text" name="password" id="pw"><br/>
    
    <input type="submit" id="submit" value="">
    
    </form>
    
    <p id="tips">

</body>

</html>

<script>

    var submit_btn = document.getElementById("submit");
    
    var xhr = new XMLHttpRequest();
    
    submit_btn.onclick = function(e){
    
        e.preventDefault();
    
        xhr.onreadystatechange = function(){
    
        if(xhr.readyState==4 && xhr.status==200){
    
            alert(xhr.responseText);
    
        }
    
        }
    
        var username = document.getElementById("un").value; //
    
        var data = "username="+username;
    
        xhr.open("post","Demo3.php");
    
        xhr.setRequestHeader("Content-Type","application/x-www-form-urlencode");
    
        xhr.send(data);
    
    }

</script>

 

//Demo3.php

<?php

    echo ($_POST);

is there a god who knows what the problem is?