Submit action message is showing when refresh the page - php

Question

When I refresh/reload the page the message(Host Name, User Name and Database Name are mandatory.) is showing. I prefer to show the message only after the submission of form.
Consider my index.php page:
// Create database
$sql = "CREATE DATABASE $dbname";
if (mysqli_query($conn, $sql)) {
echo "<div class='msg'>Database created successfully</div>";
} else {
echo "<div class='msg'>Error creating database: " . mysqli_error($conn) ."</div>";
}
mysqli_close($conn);
}else{
echo "<div class='msg'>Host Name, User Name and Database Name are mandatory.</div>";
}
}?>
<form method="post" action="index.php" class="dbform" >
<table>
<tr><td>Host Name</td><td><input type="text" name="hostname" value="localhost"></td></tr>
<tr><td>User Name</td><td><input type="text" name="username"></td></tr>
<tr><td>Password</td><td><input type="text" name="pass"></td></tr>
<tr><td>Database Name</td><td><input type="text" name="dbname"></td></tr>
<tr><td colspan="2" align="center"><input type="submit" name="subbtn" value="Create"></td></tr>
</table>
</form>
Expected bahaviour: When click the submit button, if the fields are empty show the Host Name, User Name and Database Name are mandatory. message, It is working ok.
But when I refresh the page the message is also showing. How can I solve this?

Answer 1

It's because the browser is reloading the last request you performed on that page, which is a POST request to the index.php.
When you hit reload it is again performing that same request.
Most browsers ask for a confirmation before resubmitting a post request.
What you can do solve you problem is store your messages in session and redirect to the page.
Start of your PHP code add
session_start();
Then store your messages in session
$_SESSION['msg'] = 'Your message';
Then when you have processed the current request redirect.
header('Location: YOUR_URL');
die();
Then you can check if there is any message and output
<?php if (!empty($_SESSION['msg']) : ?>
<div class='msg'><?php echo $_SESSION['msg']; ?></div>
<?php endif; ?>