javascript - Why is null displayed in the database?

After submitting with ajax, there is an additional entry in the database, but the display is null. Why is this? Am I using ajax incorrectly?

t.php

<code><script type="text/javascript">
        function ajax(url,funsucc){
            var oAjax=new XMLHttpRequest();
            oAjax.open('POST',url,true);
            oAjax.send();    
            oAjax.onreadystatechange=function(){
              if(oAjax.readyState==4){
                if(oAjax.status==200){
                  funsucc(oAjax.responseText);
                }
              }
            }
        }
    </script>
<script type="text/javascript">
        window.onload=function(){
            var oTxt=document.getElementById('txt1');
            var oBtn=document.getElementById('btn1');
            oBtn.onclick=function(){
                ajax("ajax.php",function(){
                    window.location.reload();
                });
            }
        }
    </script>
<form method="post">
    <input type="text" id="txt1" name="txt">
    <button id="btn1" type="submit">提交</button>
</form></code>

ajax.php

<code><?php
$pdo=new PDO("mysql:host=localhost;dbname=t1","root","");
$txt=$_POST["txt"];
$stmt=$pdo->prepare("insert into ajax(txt)values(?)");
$stmt->execute(array($txt));
?></code>

Reply content:

After submitting with ajax, there is an additional entry in the database, but the display is null. Why is this? Am I using ajax incorrectly?

t.php

<code><script type="text/javascript">
        function ajax(url,funsucc){
            var oAjax=new XMLHttpRequest();
            oAjax.open('POST',url,true);
            oAjax.send();    
            oAjax.onreadystatechange=function(){
              if(oAjax.readyState==4){
                if(oAjax.status==200){
                  funsucc(oAjax.responseText);
                }
              }
            }
        }
    </script>
<script type="text/javascript">
        window.onload=function(){
            var oTxt=document.getElementById('txt1');
            var oBtn=document.getElementById('btn1');
            oBtn.onclick=function(){
                ajax("ajax.php",function(){
                    window.location.reload();
                });
            }
        }
    </script>
<form method="post">
    <input type="text" id="txt1" name="txt">
    <button id="btn1" type="submit">提交</button>
</form></code>

ajax.php

<code><?php
$pdo=new PDO("mysql:host=localhost;dbname=t1","root","");
$txt=$_POST["txt"];
$stmt=$pdo->prepare("insert into ajax(txt)values(?)");
$stmt->execute(array($txt));
?></code>

It seems that you did not transfer the data to the background.

Where is the value passed?

There is no value in send. If there is a form, there is no need to write ajax. The value target="iframe_name"