PHP MySQL doesn't perform update using substring - php

Question

I am facing an issue with PHP which doesn't perform one query in my script.
The SQL query works well in my MYSQL console but nothing is happening. Year column stays NULL:
$UpdateYear='UPDATE `pat` SET `Year` = SUBSTRING(`Prepa`,7,4)';
mysqli_query($connWarehouse,$UpdateYear) or die(mysqli_error($connWarehouse));
I don't know what I am doing wrong. Here the full script:
<?php
$servername = "localhost";
$username = "root";
$password = "";
$db="datawarehouse";
// Create connection
$connWarehouse = new mysqli($servername, $username, $password, $db);
// Check connection
if ($connWarehouse->connect_error) {
die("Connection failed: " . $connWarehouse->connect_error);
}
$UpdateYear='UPDATE `pat` SET `Year` = SUBSTRING(`Prepa`,7,4)';
mysqli_query($connWarehouse,$UpdateYear) or die(mysqli_error($connWarehouse));
mysqli_close($connWarehouse));
?>

Answer 1

I finally managed to solve. I don't know if it is a correct way to do it. I have to open a new connection to the database in order to perform.
alter.php:
<?php
$servername = "localhost";
$username = "root";
$password = "";
$db="datawarehouse";
// Create connection
$connWarehouse = new mysqli($servername, $username, $password, $db);
// Check connection
if ($connWarehouse->connect_error) {
die("Connection failed: " . $connWarehouse->connect_error);
}
$AddYear='ALTER TABLE `pat` ADD COLUMN `Year` YEAR;';
mysqli_query($connWarehouse,$AddYear) or die(mysqli_error($connWarehouse));
mysqli_close($connWarehouse));
include 'uppat.php';
?>
uppat.php
<?php
require_once 'config-datawarehouse.php';
$UpdateYear='UPDATE `pat` SET `Year` = SUBSTRING(`DatePrepa`,7,4)';
mysqli_query($conn,$UpdateYear) or die(mysqli_error($conn));
mysqli_close($conn);
?>
On that way it performed the update query.