使用更新语句的未定义索引

它抛出了这个通知,因为你需要放在$newBalance = $_POST['newBalance'];里面if(isset($_POST['submit_btn'])){...}并验证它不是空的(或设置).

$newBalance="";

if(isset($_POST['submit_btn']) && !empty($_POST['newBalance']) ){

$newBalance = $_POST['newBalance'];

    $upsql = "UPDATE users SET Balance = Balance + '$newBalance' 
              WHERE Username='" . $obj->Username . "'";
    $stmt = $mysqli->prepare($upsql);
    $stmt->execute();
}

您也可以使用isset($_POST['newBalance'])而不是!empty($_POST['newBalance'])

旁注:您可能希望为按钮添加提交类型.

echo 'Top Up';

然而,它可能不是必需的; 如果您仍然遇到问题,请尝试一下.

编辑:

下

echo '
Student Username: '.$obj->Username.'
';

加

echo '';

然后在

$newBalance = $_POST['newBalance'];

加

$username = $_POST['username'];

并将您的查询修改为

$upsql = "UPDATE users SET Balance = Balance + '$newBalance' 
          WHERE Username='".$username ."'";

我的报价可能有点偏僻

echo '';

您可能需要将其更改为

echo '';

编辑#2:

由于您已经在使用会话,因此执行此操作的另一种方法是为其分配变量并将其传递给您的查询.

$username = $_SESSION['Username'];

$upsql = "UPDATE users SET Balance = Balance + '$newBalance' 
          WHERE Username='".$username ."'";

编辑#3:

你在哪里

if(isset($_POST['search'])){
$searchq = $_POST['search'];
$searchq = preg_replace("#[^0-9a-z]#i","",$searchq);

用它替换它

if(isset($_POST['search'])){
$searchq = $_POST['search'];
$searchq = preg_replace("#[^0-9a-z]#i","",$searchq);

$student = $_POST['search'];
$_SESSION['student'] = $student;

echo $_SESSION['student']; // see what echos here

然后在你的查询中,执行:

$upsql = "UPDATE users SET Balance = Balance + '$newBalance' 
          WHERE Username='".$student ."'";

如果这不起作用,我不知道还有什么可以做的,这将是进一步的帮助.我的测试是确凿的并且有效.您的查询可能会失败,此时我没有更多的想法.

基于这种情况:

$_POST['search'] = "student1";

$student = $_POST['search'];

$_SESSION['student'] = $student;

// echo $_SESSION['student'];

$student2 = $student;

echo $student2; // will echo student1