Adding data to mySQL using PHP - php

Question

I feel this should be straight forward but the data isnt going in to the table.
When I try to save the data the code executes and displays this message:
$statusMsg1 = "A problem occurred, please try again.";
Which leads me to think the problem must be with the SQL, but nothing is standing out to me to highlight what the issue is.
I changed the SQL so that insert raw text, but this produces the same message.
postCodes.php
<?php
// Form saubmission script
include_once 'submit.php';
/* Attempt MySQL server connection. */
$mysqli = new mysqli("127.0.0.1", "root", "root", "bookingpage");
// Check connection
if($mysqli === false){
die("ERROR: Could not connect. " . $mysqli->connect_error);
}
// SQL query execution
$sql = "SELECT * FROM Postcodes";
?>
<!-- Status message -->
<?php if(!empty($statusMsg1)){ ?>
<p class="stmsg"><?php echo $statusMsg1; ?></p>
<?php } ?>
<!-- GENERAL TAB -->
<p style="color:RGB(0,70,135)">You can use the text editor below to display text on your home page</p>
<form action="" method="post">
<div class="form-group">
<label for="postcode1">Enter Postcode Area</label>
<input style="font-size:12px" name="postCodetext" id="postCodetext" class="form-control required" value="e.g CF11">
</div>
<div class="form-group">
<label for="deliveryCost">Delivery Charge</label>
<input style="font-size:12px" name="costtext" id="costtext" class="form-control required" value="e.g 5.00">
</div>
<button type="button" class="save-settings btn btn-primary btn-xs"
title="<?= lang('save') ?>">
<span class="glyphicon glyphicon-floppy-disk"></span>
<?= lang('save') ?>
</button>
<input type="submit" name="submitPostCode" value="Save Data">
</form>
submit.php
<?php
// Include the database configuration file
require_once 'dbConfig.php';
$editorContent = $statusMsg = '';
$postCodeString = $statusMsg1 = '';
// SAVE POSTCODE & DELIVERY COST
if(isset($_POST['submitPostCode'])){
// Get editor content
$postCodeString = $_POST['postCodetext'];
$costString = $_POST['costtext'];
// Check whether the editor content is empty
if(!empty($postCodeString)){
// Insert editor content in the database
$insert1 = $db->query("INSERT INTO PostCodes (postCode, Cost) VALUES ('".$postCodeString."', '".$costString."')");
// If database insertion is successful
if($insert1){
$statusMsg1 = "Succeddfully Saved.";
}else{
$statusMsg1 = "A problem occurred, please try again.";
}
}else{
$statusMsg1 = 'You cannot save a blank postcode or delivery charge';
}
}