Php 为什么这段代码会生成通知：未定义索引？

这段代码并不漂亮，但我想从表

studentcourseplan

<?php
session_start();
if (!isset($_SESSION['session_username']) ) { header ("Location: login.php");exit;} //If user in not signed in, redirect to login page

include("dbconfig.php");

if (isset($_POST['course_id'])) {

    $course_id = $_POST['course_id'];
    $student_id = $_POST['student_id'];
    $start_date = $_POST['start_date'];
    $end_date = $_POST['end_date'];
    
    $studentcourseplan_update = mysql_query("
    INSERT INTO studentcourseplan (student_id,cpl_startdate, cpl_enddate, cpl_coursename, cpl_pricevalue)
      SELECT '$student_id','$start_date', '$end_date', course_name, price_value
        FROM courses c
        INNER JOIN pricegroups p ON p.price_id = c.course_price_id
      WHERE c.course_id = '$course_id'");
      
    $result = mysql_query("SELECT LAST_INSERT_ID() FROM studentcourseplan");
    $row = mysql_fetch_assoc($result);
    $cpl_id = $row['cpl_id'];
    
    $studentcourseplanelements_update = mysql_query("
    INSERT INTO studentcourseplanelements (scpe_cpl_id, scpe_name, scpe_days)
      SELECT LAST_INSERT_ID(), cp.plan_name, cp.plan_time FROM courses c
        INNER JOIN course_to_plan cpl   ON cpl.course_id = c.course_id
        INNER JOIN courseplans cp       ON cp.plan_id = cpl.plan_id
    WHERE cpl.course_id = '$course_id'");
    
    $courseinfo = mysql_query("SELECT * FROM studentcourseplan WHERE cpl_id = '$cpl_id'") or die(mysql_error());

    if ($studentcourseplan_update === true) {
        
        while ($result = mysql_fetch_assoc($courseinfo)) {
            echo $result['cpl_id'];
        }
        
    } else if ($studentcourseplan_update === false){
        echo "Kunde inte uppdatera.";
    }
}
?>

我得到：

未定义索引：第24行的\Path\To\Page.php中的cpl\u id

---

因为您在第一个select查询中只选择了

LAST\u INSERT\u ID（）

：

$result = mysql_query("SELECT LAST_INSERT_ID() FROM studentcourseplan");
$row = mysql_fetch_assoc($result);
// Doesn't exist, only $row['LAST_INSERT_ID()']
$cpl_id = $row['cpl_id'];
// Probably you meant
$cpl_id = $row['LAST_INSERT_ID()'];

但以上是不必要的。 相反，您可以只调用而不是执行查询来检索它

$cpl_id = mysql_insert_id();

关于安全的说明： 您的查询易受当前形式的SQL注入攻击。确保在所有用作查询输入的

$\u POST

变量上调用

mysql\u real\u escape\u string（）

$course_id = mysql_real_escape_string($_POST['course_id']);
$student_id = mysql_real_escape_string($_POST['student_id']);
$start_date = mysql_real_escape_string($_POST['start_date']);
$end_date = mysql_real_escape_string($_POST['end_date']);

从长远来看，建议您远离
mysql_*（）
函数，转而学习使用支持PDO或MySQLi等预处理语句的更现代的API

$course_id = mysql_real_escape_string($_POST['course_id']);
$student_id = mysql_real_escape_string($_POST['student_id']);
$start_date = mysql_real_escape_string($_POST['start_date']);
$end_date = mysql_real_escape_string($_POST['end_date']);