Query working on phpmyadmin but not in php script - php

Question

I already tried other solutions from similar questions but it seems that nothing worked. Like the title says, the query I need to execute on my db works fine on phpMyAdmin, but it doesn't work when used inside a php function. This is the code in common_functions.php:
function eliminaRistorante($id_ristorante) {
global $conn;
// query su ristorante
$sql = "DELETE FROM ristorante WHERE id_ristorante = $id_ristorante";
echo $sql;
if($conn->query($sql) === TRUE) {
return true;
} else {
echo "Error: " . $sql . "<br>" . $conn->error;
return false;
}
}
This is the code in hold.php:
include('includes/logic/common_functions.php');
if (isset($_POST['del'])) {
$id_ristorante = $_POST['del'];
eliminaRistorante($id_ristorante);
header('location: admin/ristorante.php');
}
I put an echo statement to see if the query was effectively correct.
This is the output:
DELETE FROM ristorante WHERE id_ristorante = 5
Fatal error: Uncaught Error: Call to a member function query() on null in C:\xampp\htdocs\projects\db_ristorante\includes\logic\common_functions.php:57 Stack trace: #0 C:\xampp\htdocs\projects\db_ristorante\hold.php(8): eliminaRistorante('5') #1 {main} thrown in C:\xampp\htdocs\projects\db_ristorante\includes\logic\common_functions.php on line 57
Lastly, I tried these solutions:
MySql query not in working in PHP but works in phpMyAdmin
Inside that question the user kindly shared other solutions that I considered too.

Answer 1

Uncaught Error: Call to a member function query() on null in
your $conn Object is null. You did not initialise a PDO instance.
Here is a introduction, on how to do that, PHP The Right Way - PDO