このコードはきれいではありませんが、最後に生成された自動インクリメント ID をテーブルから出力したいと考えています。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.";
    }
}
?>
24行目で泣き言を言います。これは次のとおりです。
$cpl_id = $row['cpl_id'];
私は得る:
未定義のインデックス: 24 行目の \Path\To\Page.php の cpl_id