无法使用下拉选项从表中获取数据

Question

我无法从表中获取数据

   <?php

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

     $i=0;
     $radio = 0;
     $semester = '';
     $all_query = mysqli_query($conn,"select * from student_tbl where semester='$semester' ");

     while ($data = mysqli_fetch_array($all_query)) {
       $i++;
     ?>

   <?php

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

     $i=0;
     $radio = 0;
     $semester = '';
     $all_query = mysqli_query($conn,"select * from student_tbl where semester='$semester' ");

     while ($data = mysqli_fetch_array($all_query)) {
       $i++;
     ?>

错误消息：

  

警告：mysqli_query（）：无法在146行的C：\ xampp \ htdocs \ come \ attendance \ teacher \ attendance.php中获取mysqli

     

警告：mysqli_fetch_array（）期望参数1为mysqli_result，在第148行的C：\ xampp \ htdocs \ come \ attendance \ teacher \ attendance.php中给出空值

预期结果：

从表中获取数据