Borrowing and Returning Function in PHP/MySQL

<?php
require ('include/database.php');
 
require ('session.php');
 
$id = $_POST['selector'];
$client_id = $_POST['client_id'];
 
if ($id == '')
	{
	header("location: release.php");
?>
 
 
	<?php
	}
  else
	{
	mysql_query("insert into tbl_release (client_id,date_borrow) values ('$client_id',NOW()) ") or die(mysql_error());
	$query = mysql_query("select * from tbl_release order by release_id DESC") or die(mysql_error());
	$row = mysql_fetch_array($query);
	$release_id = $row['release_id'];
	$history_record = mysql_query("select * from user where user_id=$id_session");
	$row1 = mysql_fetch_array($history_record);
	$user = $row1['firstname'] . " " . $row1['lastname'];
	$N = count($id);
	for ($i = 0; $i < $N; $i++)
		{
		mysql_query("INSERT INTO transaction_history (item_id,action,client_id,release_id,admin_name,date_added) VALUES ('$id[$i]','Borrowing Item','$client_id','$release_id','$user',NOW())") or die(mysql_error());
		mysql_query("insert into release_details (item_id,release_id,release_status) values ('$id[$i]','$release_id','pending')") or die(mysql_error());
		}
 
	header("location: release.php");
	}
 
?>