add-invoice.php
<?php include 'includes/header.php';
include ('includes/connection.php');
extract($_POST);
$id = rand(00,99);
$Invoice_Number = rand(0000,9999);
$date = date("m/Y");
if (isset($_POST['submit'])) {
$Client_Name = $_POST['Client_Name'];
$Item1 = $_POST['Item1'];
$Price1 = $_POST['Price1'];
$Quantity1 = $_POST['Quantity1'];

$totalClient_Name = sizeof($Client_Name);

for($i=0;$i<$totalClient_Name;$i++) {

$Client_Name = $Client_Name[$i];
$Item1 = $Item1[$i];
$Price1 = $Price1[$i];
$Quantity1 = $Quantity1[$i];
$sql = mysqli_query($connection, "INSERT INTO invoice (id, Invoice_Number, Client_Name, Item1, Price1, Quantity1, monthyear)VALUES ('$id', '$Invoice_Number', '$Client_Name', '$Item1', '$Price1', '$Quantity1', '$date')");
}
if (mysqli_num_rows($sql) > 0) {
header("Location: invoice.php");
}
header("Location: invoice.php");
}
?>
//----------------------//
header.php
<?php
session_start();
include 'includes/connection.php';
extract($_SESSION);

if (isset($_SESSION['username'])) {
# code...
}else{
header("Location: index.php");
}

?>