Create Concat Trigger in Phpmyadmin

So let post my code first and then i'll explain my motivation and another issue that has presented itself that i'm trying to resolve.

?php 
if (isset($_POST['submit'])) {
    $mfr   = $_POST['mfr'];
    $type  = $_POST['type'];
    $desc  = $_POST['description'];
    $price = $_POST['price'];
    $qty   = $_POST['qty'];

    $last_id = 0;
    $sql = "SELECT m_id FROM merchandise";
    $result = mysqli_query($connection, $sql);
    while($row = mysqli_fetch_assoc($result)) {
    $last_id = $row["m_id"];
    }
    $next_id = $last_id+1;
    $conc = number_format($next_id/100,2,'-','');

    $query = "INSERT INTO merchandise (mfr,type,description,mer_sku,price,qty) ";
    $query .="VALUES ('$mfr','$type','$desc','MR{$mfr}{$conc}','$price','$qty')";
    $add_sku_query = mysqli_query($connection, $query);
    confirmQuery($add_sku_query);
    $alert = <<<DELIMETER

<div class='alert alert-warning alert-dismissible fade in' role='alert'>
<button type="button" class="close" data-dismiss="alert" aria-label="Close">
    <span aria-hidden="true">&times;</span>
</button>
<strong>Inventory Updated!</strong> <a href='inventory.php?view_all_inventory'>View All Inventory</a>
</div>
DELIMETER;

    echo $alert;
    }
?>

So first off let me point out that I did try to set this up using PDO but i'm not yet fully comprehending it so i'm sticking with mysqli. Which i know that how I have the code setup it leaves it vulnerable to SQL Injection.

So that aside, My motivation was to try and add items into a table sequentially. However, my code currently auto increments without regards to the $mfr variable which is ESSENTIAL. Each manufacturer has a unique value and so the auto increment would NEED to auto increment PER manufacturer instead of auto increment while disregarding the the manufacturer variable that is concatenated.

**CURRENT AUTO INCREMENT, [NOT WHAT I WHAT]**
MR500-01
MR600-02
MR700-03
MR500-04
MR600-05
MR700-06

**IDEAL AUTO INCREMENT, [WHAT I WANT]**
MR500-01
MR600-01
MR700-01
MR500-02
MR600-02
MR700-02

I hope I explained my thought. Anyways, this is my current status and I'm working towards creating the resolution that I NEEED.

Hope this was somewhat helpful to anyone or if someone has some helpful input that would be great and much appreciated. Thanks.

Instead of GUI, try running it in the query section. I think the error is because mysql treats ; as the end of your trigger declaration.

So redefine delimiter before writing code for the trigger

Try the below code

delimiter |

CREATE TRIGGER `merch_label` 
BEFORE INSERT ON `merchandise` 
FOR EACH ROW  
BEGIN  
    SET NEW.mer_sku = CONCAT(NEW.mer_sku, 'MR'); 
END;
|

delimiter ;