尝试使用绝对路径，替换为：

$target_dir = "uploads/";

比如：

$target_dir = __DIR__ . '/uploads/';

括号设置不正确，并且缺少表单标记。应在发布事件后通过单击“提交”按钮声明所有内容：

$imageFileType = "";
$target_file = "";

if(isset($_POST["submit"])) {
$target_dir = "uploads/";
$target_file = $target_dir . basename($_FILES["fileToUpload"]["name"]);
$uploadOk = 1;
$imageFileType = strtolower(pathinfo($target_file,PATHINFO_EXTENSION));
$check = getimagesize($_FILES["fileToUpload"]["tmp_name"]);

if($check !== false) {
  echo "File is an image - " . $check["mime"] . ".<br>";
  $uploadOk = 1;
} else {
  echo "File is not an image.";
  $uploadOk = 0;
}

// Check if file already exists
if (file_exists($target_file)) {
  echo "Sorry, file already exists.";
  $uploadOk = 0;
}

// Check file size
/*if ($_FILES["fileToUpload"]["size"] > 500000) {
  echo "Sorry, your file is too large.";
  $uploadOk = 0;
}*/

// Allow certain file formats
if($imageFileType != "jpg" && $imageFileType != "png" && $imageFileType != "jpeg" && $imageFileType != "gif" ) {
  echo "Sorry, only JPG, JPEG, PNG & GIF files are allowed.";
  $uploadOk = 0;
}

// Check if $uploadOk is set to 0 by an error
if ($uploadOk == 0) {
  echo "Sorry, your file was not uploaded.";
// if everything is ok, try to upload file
} else {
if (move_uploaded_file($_FILES["fileToUpload"]["tmp_name"], $target_file)) {
  echo "The file ". htmlspecialchars( basename( $_FILES["fileToUpload"]["name"])). " has been uploaded.";
} else {
  echo "Sorry, there was an error uploading your file.";
}
}
}
print "<FORM  action='{$_SERVER['PHP_SELF']}'  method='POST' enctype='multipart/form-data'>";
?>
<input type="file"  name="fileToUpload"/>
<?php
print "<button type='submit' name='submit' style='width:100px;'>upload</button>";
print "</form>";