如何使用move_uploaded_file调试问​​题？

<form action="upload.php" method="post" enctype="multipart/form-data">
 <input type="file" name="fileToUpload" id="fileToUpload">
 <input type="submit" value="Upload File" name="submit">
</form>

if($_FILES["fileToUpload"]["error"] > 0) {
  echo "Error: " . $_FILES["fileToUpload"]["error"] . "<br>";
} else {
  // 文件上传成功
}

$target_dir = "uploads/";
$target_file = $target_dir . basename($_FILES["fileToUpload"]["name"]);

if (move_uploaded_file($_FILES["fileToUpload"]["tmp_name"], $target_file)) {
  echo "The file " . htmlspecialchars(basename($_FILES["fileToUpload"]["name"])) . " has been uploaded.";
} else {
  echo "Sorry, there was an error uploading your file.";
}