Refresh the aside without new page - php

Question

So i have the following code for the aside which allows the user to search for other user(NOTE: i have left out the aside-tag here in order to make the code more readable):
<form action="index.php" method="POST">
<input id="search" type="text" placeholder="Search for Friends" name="search_name">
<input class="submit" type="submit" name="search-submit" value="Search">
</form>
After than i get the data from the database and display them in a variable whicht is called $output:
<?php
echo #$output;
?>
Ok so now when the user clicks on a name it should load the chatwindow in the aside instead of the search results. So the $output should be replaced with the chat window which has the following code:
<?php
if (isset($_POST['chat-submit'])) {
echo '<form method="POST" action="index.php" class="form-back">
<input class="back" type="submit" name="back" value="←Back">
</form>
<form>
<div class="chatwith"><h1><?php echo $username; echo ":"; ?></div>
<?php echo $finalstatus; ?>
<form action="index.php" method="POST" class="message-form">
<input class="message" type="text" placeholder="message" name="message">
<input class="submit" type="submit" name="message-submit" value="Send">
</form>';
}
?>
ok so both codes are working fine but when i click on one of the $outputs (and activate "chat-submit") than the old form from the search box is still there. How can i remove the old form so that only the chatwindows will be shown. Is there some kind "refresh only the aside"? If you have any questions please ask me.

Answer 1

If you set $refresh to 0 you dont need the if. Because it is always 0. Do like this:
<?php
if(isset($_POST['chat-submit'])) {
echo '//here goes the chat-form but this is too long for this comment. But its the same as above'; }
else {
echo $output;
}
?>

Answer 2

So it does work now this is the right solution from #Dieter Kräutl:
<?php
if(isset($_POST['chat-submit'])) {
echo '<form method="POST" action="index.php" class="form-back">
<input class="back" type="submit" name="back" value="←Back">
</form>
<form>
<div class="chatwith"><h1><?php echo $username; echo ":"; ?></div>
<?php echo $finalstatus; ?>
<form action="index.php" method="POST" class="message-form">
<input class="message" type="text" placeholder="message" name="message">
<input class="submit" type="submit" name="message-submit" value="Send">
</form>'; }
else {
echo $output;
}
?>