PHP develops simple book borrowing system homepage book return function

As shown in the picture

After you have borrowed a book, there is an operation directory under the operation column on the main page that displays "You have borrowed the book and I want to return it"

Click "I want to return the book" to realize the function of returning the book.

After returning the book, the "current quantity" of the book on the main page will be increased by one.

First of all, it is necessary to determine whether the user is logged in.

<?php
if ($_SESSION['id']==""){
   echo "<script language=javascript>alert('您还没有登陆');window.location='landing.php'</script>";
   exit();
}
?>

Display returned books through SQL statements. If the book is returned successfully, the borrowing record of the book will be deleted from the borrowing table, and the number of books will be increased by one in the homepage display table.

<?php
$book_id=$_GET["book_id"];
//构建sql语句还书
//在lend表中删除该借书记录
$returnsql="DELETE FROM lend where book_id='$book_id' and user_id=".$_SESSION['id'];
mysqli_query($link,$returnsql) or die ("删除借书记录失败：".mysqli_errno($link));
//在book表中增加一本现存书数量
$booksql="UPDATE yx_books SET leave_number=leave_number+1 where id='$book_id'";
mysqli_query($link,$booksql) or die ("增加剩余书数量失败：".mysqli_error($link));
echo "<script language=javascript>alert('还书成功');window.location='index.php'</script>";
?>