移动到函数时查询不起作用

我在index.php页面上有一个查询，它运行得很好。但是，当我将查询移到functions.php，创建完全相同的查询时，它会中断。知道为什么吗？

index.php

<ul>
 <?php $cms_sub_pages = cms_find_sub_pages ($file_name); ?>
 <?php while($subpage = mysqli_fetch_assoc($cms_sub_pages)) { ?>
   <li><a href="/edit.php?
       <?php echo $subpage["page_id"]; ?>"><?php echo $subpage["link_name"]; ?></a
   </li>    
 <?php
 }
 ?>
 <?php mysqli_free_result($sub_pages); ?>
 </ul>

functions.php

function cms_find_sub_pages ($file_name) {
    global $conn;
    $query = "SELECT * FROM pages WHERE main_cat = 2 AND category =  '{$file_name}' ORDER BY subcategory_order ASC ";
    $sub_pages = mysqli_query($conn, $query);
    confirm_query($sub_pages);
    return $sub_pages;  
}

解决：问题出现在index.php中：

<?php $cms_sub_pages = cms_find_sub_pages ($file_name); ?>

更改为：

<?php $cms_sub_pages = cms_find_sub_pages ($mainpage["page_id"]); ?>