Need to set variables to "Null" after POST action (PHP function) - php

Question

I have a comments section in a website that uses a "setComments" function to enter data into a SQL DB. This works great, but I found that if I refresh the page, the comment is re-entered into the DB and displayed as multiple comments using a "getComments" function.
I know I need to drop the values of the comment variables but tried a few places in the code and no joy.
This is the form to enter the comment:
<?php
echo "<form method='POST' action='".setComments($conn)."'>
<input type='hidden' name='uid' value='Anonymous'>
<input type='hidden' name='pid' value='$post_id'>
<input type='hidden' name='date' value='".date('Y-m-d H:i:s')."'>
<label>Add Comment</label>
<textarea name='message'class='form-control' width='50' cols='40'
rows='3'></textarea><br>
<button type = 'submit' name ='commentSubmit' class='btn btn-
primary'>Submit<span class='glyphicon glyphicon-chevron-right'></span>
</button>
</form>";
getComments($conn)
?>
<?php
function setComments($conn){
if (isset($_POST ['commentSubmit'])) {
$uid=$_POST['uid'];
$pid=$_POST['pid'];
$date=$_POST['date'];
$message=$_POST['message'];
$sql = "INSERT INTO comments (uid, pid, date, message)
VALUES('$uid','$pid', '$date', '$message')";
$result = $conn->query($sql);
}
}
function getComments($conn) {
if (isset($_GET['post'])) {
$pid = $_GET['post'];
$sql = "SELECT * FROM comments WHERE pid=$pid";
$result = mysqli_query($conn, $sql) or die(mysqli_error($conn));
if (mysqli_num_rows($result) > 0) {
while($row = $result->fetch_assoc()){
echo "<div class='comment-box'>";
echo $row['uid']."<br>";
echo $row['date']."<br>";
echo $row['message'];
echo "</div>";
}
}
else {
echo "Be the first to comment";
}
}
}
Comments are entered into sql DB fine, but variables not discarded once complete so they are re-entered into DB and displayed again on page refresh (page containing the comment form also displays the comments.

Answer 1

I ended up using the following script in the page - it seems to work fine:
<script>
if ( window.history.replaceState ) {
window.history.replaceState( null, null, window.location.href );
}
</script>