$_SESSION display not reflecting changes after hitting submit once - php

Question

I'm creating a profile page where the name of the user can be edited. I can't seem to figure out how to update the name of the user that is displayed at the top page (i.e. Hi Kana!) after a successful edit.
The name is generated using a session variable at the top of my page, $_SESSION['username']
<?php
session_start();
if(isset($_POST['submit']) && isset($_SESSION['newusername'])){
unset($_SESSION['username']); //clear existing username session
$_SESSION['username'] = $_SESSION['newusername']; //replace with the new username
echo $_SESSION['username'];
} else {
echo $_SESSION['username'];
}
?>
After hitting the submit button with a different name set on the text input it should change but it does not. These are the lines of codes after the code I mentioned above.
<?php
$textinput = $_POST['textinput'];
if(isset($_POST['submit']))
{
if(strlen($_POST['textinput']) < 2)) {
$errormessage = 'too short';
}
else
{
$_SESSION['newusername'] = $textinput; //setup a new session for the new username to be used above.
}
}
?>
Note that the two quoted set of codes are in the same page.
What happens is that the name still shows the previous name, which is supposedly overwritten. I tried refreshing the page but it still does not change, I need to submit again (2nd time) to show the new name.

Answer 1

You can and should drastically simplify your code. Currently there are a variety of unnecessary operations here. Consider semantically what you want to accomplish:
If the user submits their name, update the session with the new value. Then output the value.
That's it. Let the semantics of what you want to accomplish guide how you accomplish it. Start the session, check for the form post, conditionally update the value, output the current state of the value. For example:
<?php
session_start();
// update the value
if(isset($_POST['submit'])) {
$_SESSION['username'] = $_POST['textinput'];
}
// output the value
echo $_SESSION['username'];
?>
Maybe you'll want to add a sensible default in the absence of a username. That would just be something like:
<?php
session_start();
// set a default value
if (!isset($_SESSION['username'])) {
$_SESSION['username'] = "Default User";
}
// update the value
if(isset($_POST['submit'])) {
$_SESSION['username'] = $_POST['textinput'];
}
// output the value
echo $_SESSION['username'];
?>