浏览器不响应标题重定向

please help... i am new to php. Thanks

php1 is to send a data ('1234') via method Post to php2. php2 is supposed to redirect using Header Location to php3 with a data ('invalid').

Developer Tools of Chrome indicate that everything went well (Post data sent and received. Get data sent and received). Somehow, the browser does not response and stay at php1. I have tried Safari and Firefox. No response. Would be really grateful if you could advise. Thanks

The 3 php files are:

php1

<?php 
session_start();
$M = '';
if (isset($_GET['m'])) {
    $M = $_GET['m'];
}
?>
<!DOCTYPE html>
<html>
<head>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script>
$(document).ready(function(){
    $('#send').click(function () {
      var str = '1234';
      $.post('php2.php',
        {
        email: str
        },
        function (data, status) {
        }
      );
    });
});
</script>
</head>
<body>
<div>
<button id="send">SEND</button>
<br>
<?php echo $M; ?>
</div>
</body>
</html>

php2

<?php
session_start();
ob_start();
error_reporting(E_ALL);
if (!empty($_POST)){
    $Email = $_POST['email'];
    if (!filter_var($Email, FILTER_VALIDATE_EMAIL)) {
        header('Location: php3.php?m=invalid');
        exit();
    } 
} else {
    header('Location: php1.php?m=nodata');
    exit();
}
ob_end_flush();
?>

php3

<?php 
session_start();
$M = '';
if (isset($_GET['m'])) {
    $M = $_GET['m'];
}
?>
<!DOCTYPE html>
<html>
<head>
</head>
<body>
<div>
<?php echo $M; ?>
</div>
</body>
</html>

Here is a screenshot of chrome developer:

Screenshot of Chrome Developer .Tools

You should change the code for php1.php like this

<?php 
session_start();
$M = '';
if (isset($_GET['m'])) {
    $M = $_GET['m'];
}
?>
<!DOCTYPE html>
<html>
<head>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script>
$(document).ready(function(){
    $('#send').click(function () {
      var str = '1234';
      $.post('php2.php',
        {
        email: str
        },
        function (data, status) {
          $("#content").html(data); // <----- ADDED
        }
      );
    });
});
</script>
</head>
<body>
<div id="content">
<button id="send">SEND</button>
<br>
<?php echo $M; ?>
</div>
</body>
</html>