如何在 MySQLi 中使用 IN 子句绑定 UNION 中的结果？

我正在尝试使用 IN 子句编写 UNION prepare 语句的查询。我正在使用 call_user_func_array() 绑定数组，但没有显示结果。我认为绑定数组没有问题。

这是我的代码

PHP代码：

    if(isset($_GET['subject'])) //subject is array of subjects
    {

    $search1 = $_GET['subject'];

    echo $subject_count = count($search1);

    echo $subject_csph = implode(',', array_fill(0, $subject_count, '?'));  // comma-separated placeholders

    $total_count = $subject_count * 14;
    $total_params = array_merge($search1, $search1, $search1, $search1, $search1,
                            $search1, $search1);

    $param_string = str_repeat('s', $subject_count * 14);  // write s chars before i chars


    $construct .="(subject in ($subject_csph) or subject in ($subject_csph)
          or subject in ($subject_csph) or subject in ($subject_csph) or subject in ($subject_csph)
          or subject in (subject_csph) or subject in (subject_csph))";

    $stmt = $conn->prepare("(SELECT id,year,name,city FROM student1 WHERE $construct) union all (SELECT id,year,name,city FROM student2 WHERE $construct)"); 

    if($stmt){

    array_unshift($total_params, $param_string);  // prepend the type values string

    $ref = [];  // add references
    foreach ($total_params as $i => $v) {
        $ref[$i] = &$total_params[$i];  // pass by reference as required
    }
     call_user_func_array([$stmt, 'bind_param'], $ref);    

      $stmt->execute();
      $stmt1->bind_result($id,$year,$name,$city,$id,$year,$name,$city);

      $count = 0;
      while($stmt1->fetch()){
      echo $id;
      $count++;
        }
        echo $foundnum=$count; 
       }
    }

错误日志中没有错误。我在这里做什么错了？