This line:
mysqli_query("DELETE FROM registered WHERE id=$id",$db);
the connection comes first in mysqli_ and not last.
mysqli_query($db, "DELETE FROM registered WHERE id=$id");
mixed mysqli_query ( mysqli $link , string $query [, int $resultmode = MYSQLI_STORE_RESULT ] )
You could also do it all in one go, without using mysqli_select_db
$db = mysqli_connect("localhost", "root", "", "mydb");
You should also use conditional statements along with isset() and empty().
Also make sure the id being passed through is an int. Otherwise, you will need to quote it.
I.e.:
mysqli_query($db, "DELETE FROM registered WHERE id='$id'");
Sidenote: Your present code is open to SQL injection. Use mysqli with prepared statements, or PDO with prepared statements, they're much safer.
Make use of error reporting/checking for both PHP and MySQL.
Consult:
Edit:
Do the following:
mysqli_query($db, "DELETE FROM registered WHERE id='$id'")
or die(mysqli_error($db));
to see if errors come of it from your query.
Add error reporting to the top of your file(s) which will help find errors.
<?php
error_reporting(E_ALL);
ini_set('display_errors', 1);
Edit #2:
Replace:
$id=$_POST['idelete'];
mysqli_query("DELETE FROM registered WHERE id=$id",$db);
with:
if(!empty($_POST['idelete'])){
$id=$_POST['idelete'];
}
$query = "DELETE FROM registered WHERE id=$id";
$result = mysqli_query($db, $query);
if ( !$result ) {
trigger_error('query failed', E_USER_ERROR);
}
and see if any errors come of it.
