我已经用2个图像（2个输入字段）和1个pdf文件在同一时间设置了一个例子
因此，我假设您的html标记中包含所有文件输入，如下所示：

<input type="file" name="PDF">
<input type="file" name="MainImage">
<input type="file" name="SecondImage[]">

在这种情况下，要将上载文件的文件路径插入数据库，应该执行以下操作：

$uploadMainTo = null;
if(isset($_FILES['MainImage'])){
  $main_image_name = $_FILES['MainImage']['name'];
  $main_image_size = $_FILES['MainImage']['size'];
  $main_image_tmp = $_FILES['MainImage']['tmp_name'];
  $uploadMainTo = $uploadLocation.$main_image_name;
  $moveMain = move_uploaded_file($main_image_tmp,$uploadMainTo);
}

$uploadSecondTo = array();
if(count(array_filter($_FILES['SecondImage']))>0){
  foreach(array_filter($_FILES['SecondImage']) as $value){
   $second_image_name = $value['name'];
   $second_image_size = $value['size'];
   $second_image_tmp = $value['tmp_name'];
   $uploadSecondTo[] = $uploadLocation.$second_image_name;
   $moveSecond = move_uploaded_file($second_image_tmp,$uploadSecondTo);
  }
  $uploadSecondTo = implode(',',$uploadSecondTo);
}

$uploadPdfTo = null;
if(isset($_FILES['PDF'])){
  $pdf_name = $_FILES['PDF']['name'];
  $pdf_size = $_FILES['PDF']['size'];
  $pdf_tmp = $_FILES['PDF']['tmp_name'];
  $uploadPdfTo = $uploadLocation.$pdf_name;
  $movepdf = move_uploaded_file($pdf_tmp,$uploadPdfTo);
}

$query = $db->execute("INSERT INTO users (pdf, main_image, second_image) VALUES (?,?,?) WHERE ID = ?", array($uploadPdfTo, $uploadMainTo, $uploadSecondTo, $user_id) );

我只是不确定是否必须用null或'null'初始化$uploadxto变量。如果你有问题，请这样测试。
注意：不要忘记在表单上使用enctype=“multipart/form data”
希望这对你有帮助