在这里，现在准备好了查询：

lib4.php文件

<form action='add_borrowing1.php' method='post'>
          <br>
          <select name="student">
        <?php
          $link = mysqli_connect("localhost", "root", "", "library");
          $sql = "SELECT DISTINCT studentId, studentName
          FROM students";
          $result = mysqli_query($link, $sql);
          print "<option value='students' selected>Students</option>";
          While ($row = mysqli_fetch_assoc($result))
            {
            print "<option value=" . $row["studentId"] . ">"
          . $row["studentName"] . "</option>";
            }
        ?>
          </select>
                    <p>Book:</p>
          <br>
          <select name="book">
        <?php
          $link = mysqli_connect("localhost", "root", "", "library");
          $sql = "SELECT DISTINCT bookId, bookName
          FROM books";
          $result = mysqli_query($link, $sql);
          print "<option value='books' selected>Books</option>";
          While ($row = mysqli_fetch_assoc($result))
            {
            print "<option value=" . $row["bookId"] . ">"
          . $row["bookName"] . "</option>";
            }
        ?>
          </select>
          <input type="submit" />
          </form>

          <?php
          if (!isset($_GET["raw"])) {
            echo "";
          }
          else {
            echo "Borrowing has been successfully added!";
          }

?>

add_borrowing.php
======================
<?php
$book = $_POST["book"];
$student = $_POST["student"];
$link = mysqli_connect("localhost", "root", "", "library");
        $sql = "INSERT INTO borrowing (bookId, studentId) VALUES (?,?)";

$stmt = mysqli_prepare($link, $sql);

            /*
            Explanation:
            i Integers
            d Doubles
            b Blobs
            s Everything Else 
            */
            mysqli_stmt_bind_param($stmt, "ii", $book,
                                   $student);
            mysqli_stmt_execute($stmt);
                mysqli_stmt_close($stmt);
                mysqli_close($link);

header("Location:lib4.php?raw=1");
?>