Description:
------------
PHP version : 8.0.6

It seems that when you fetch the file using POST, if the file uses session_start, it will not render or run the .php file. The page will only load when you submit the form without using js fetch.


Test script:
---------------
[index.php]
<script>
fetch('login.php', {method : "POST", body : JSON.stringify({test : "test"})});
</script>

[login.php]
<?php
session_start();
session_write_close(); # without this is also cause the same issue
echo 'testing';
?>

Expected result:
----------------
The rendered result from fetch.

Actual result:
--------------
Nothing was shown from inspect_element > network.