PHP Form not connecting to MYSQL Database

Question

I haven't done anything with PHP or MYSQL since last year and I was still only a beginner then, so please forgive the question. I'm sure I am missing something so simple and obvious but for the life of me can't figure it out, so I hope someone can point me in the right direction.

Here is my set up: I have a db_connect.php page where my Database connection details are

$dbc = new mysqli('localhost', 'root', '', 'db_name');

//run connect_errno to enure it connects to the db. If not then kill the rest of the script and show error message
if($dbc->connect_errno) {
    die('Failed to connect to the MYSQL Database');
}

I have a add_stocktake.php page with the following form:

<div class ="wrapper">
 <?php if (isset($_GET["status"]) AND $_GET["status"] == "success") { ?>
         <p> Your Successfully added a new stock take. </p>
 <?php } else { ?> 



            <form method="post" action="add_stocktake.php">
                <table>
                    <tr>
                        <th>
                            <label for="manufacturer">Manufacturer</label>
                        </th>
                        <td>
                            <input type="text" name="manufacturer" id="manufacturer">
                        </td>
                        <th>
                            <label for="model">Model</label>
                        </th>
                        <td>
                            <input type="text" name="model" id="model">
                        </td>
                        <th>
                            <label for="product_name">Product Name</label>
                        </th>
                        <td>
                            <input type="text" name="product_name" id="product_name">
                        </td>
                        <th>
                            <label for="quantity">Quantity</label>
                        </th>
                        <td>
                            <input type="text" name="quantity" id="quantity">
                        </td>
                    </tr>
<tr>
                        <th>
                            <label for="stocktake_date">Date of Stock take</label>
                        </th>
                        <td>
                            <input type="date" name="stocktake_date" id="stocktake_date">
                        </td>
</table>
                <p>
                <input type="submit" value="Save">
            </form>
         <?php }?> 
</div>

and the following php:

<?php
require 'db_connect.php';

if ($_SERVER["REQUEST_METHOD"] == "POST") {
    $manufacturer = trim($_POST["manufacturer"]);
    $model = trim($_POST["model"]);
    $product_name = trim($_POST["product_name"]);
    $quantity = trim($_POST["quantity"]);
    $stocktake_date = trim($_POST["stocktake_date"]);

    //VALIDATIONS

    if ($manufacturer == "" OR $model == "" OR $product_name == "" OR $quantity == "" OR $quantity == "" OR $stocktake_date == ""){
        $error_message = "You must fill in all the fields.";
    }

// if there's not a previous error message run a database query to add information to the database
    if (!isset ($error_message)) {
        $query =
        $sql = "INSERT INTO stocktake_tbl ('manufacturer','model','product_name','quantity','stocktake_date') VALUES ('".$manufacturer."','".$model."','".$product_name."','".$quantity."','".$stocktake_date."')";
          $query_run = mysqli_query($dbc, $sql);
            echo "This has been saved successfully";
    }

}
?>

Here is what has happens: When I load the page first time and click the save button with nothing entered the page is just refreshed (no error messages). When I fill in the information and click save the page refreshes and it says the "This has been saved successfully" message. When I go to PHPmyadmin there has been nothing added. I added some values while I was at myadmin and then did a query within my page to see if I could pick up the information and it just returned a blank screen so I know it must something I have or haven't done correctly in regards to connecting it up.

Any advice is greatly appreciated (will teach me for leaving it alone for so long!)

Thanks