// Admin sees pending requests
$pending = $conn->query("SELECT * FROM payment_requests WHERE status = 'pending'");

// Admin clicks "Approve"
if(isset($_GET['approve'])) {
    $id = $_GET['approve'];
    // 1. Update status
    $conn->query("UPDATE payment_requests SET status = 'approved' WHERE id = $id");
    // 2. Add Balance to User
    $req = $conn->query("SELECT * FROM payment_requests WHERE id = $id")->fetch_assoc();
    $conn->query("UPDATE users SET balance = balance + {$req['amount']} WHERE id = {$req['user_id']}");
    echo "Balance Approved!";
}