I tried to delete user from mysql database with this code

if (isset($_POST['user_delete'])) {
    $key = $_POST['keyToDelete'];
    $check = "DELETE FROM user WHERE id = ". $key or die(mysqli_error($connection));

    $result2 = $connection->query($query);
    if($result2->num_rows >0){
         $query_delete = "DELETE FROM user WHERE id =". $key or die(mysqli_error($connection));

    var_dump($query_delete);
} else {

}

but it don't want to delete my database. but the sql already right and I also got the id because I tried to var_dump it. please help what was wrong with my code

You have a few issues here,

1. Your or die(mysqli_error($connection)) is to the querystrings, not the actual queries. Besides, instead of manually checking for errors it's much better to configure to throw errors automatically. For this add the following line to the connection code:

   mysqli_report(MYSQLI_REPORT_ERROR | MYSQLI_REPORT_STRICT);
   

2. You attempt to delete it twice? Though the second query is never executed, you just define the querystring (and never run it).

3. num_rows is only usable on select-statements. You want affected_rows to check if the query actually deleted any data.
4. You're not using a prepared statement.

if (isset($_POST['user_delete'])) {
    $key = $_POST['keyToDelete'];
    $query = "DELETE FROM user WHERE id = ?";
    $stmt = $connection->prepare($query);
    $stmt->bind_param("s", $key);
    $stmt->execute();
    if ($stmt->affected_rows) {
        echo "Deleted ".$stmt->affected_rows." rows";
    } else {
        echo "No rows matched the criteria.";
    }
    $stmt->close();
}