Move_uploaded_file()函数不起作用

<?php
if ($_SERVER['REQUEST_METHOD'] == 'POST') {
    $target_dir = "/path/to/target/directory/";
    $target_file = $target_dir . basename($_FILES["file"]["name"]);
    $uploadOk = 1;

    // 检查文件是否已经存在
    if (file_exists($target_file)) {
        echo "Sorry, file already exists.";
        $uploadOk = 0;
    }

    // 检查文件大小
    if ($_FILES["file"]["size"] > 500000) {
        echo "Sorry, your file is too large.";
        $uploadOk = 0;
    }

    // 允许特定文件格式
    $imageFileType = strtolower(pathinfo($target_file,PATHINFO_EXTENSION));
    if($imageFileType != "jpg" && $imageFileType != "png" && $imageFileType != "jpeg"
    && $imageFileType != "gif" ) {
        echo "Sorry, only JPG, JPEG, PNG & GIF files are allowed.";
        $uploadOk = 0;
    }

    // 检查是否为真实图片
    if(isset($_POST["submit"])) {
        $check = getimagesize($_FILES["file"]["tmp_name"]);
        if($check !== false) {
            echo "File is an image - " . $check["mime"] . ".";
            $uploadOk = 1;
        } else {
            echo "File is not an image.";
            $uploadOk = 0;
        }
    }

    // 检查临时目录是否可写
    $temp_dir = ini_get('upload_tmp_dir');
    if (!is_writable($temp_dir)) {
        echo "Sorry, the temporary directory is not writable.";
        $uploadOk = 0;
    }

    // 尝试移动文件
    if ($uploadOk == 0) {
        echo "Sorry, your file was not uploaded.";
    } else {
        if (move_uploaded_file($_FILES["file"]["tmp_name"], $target_file)) {
            echo "The file ". htmlspecialchars( basename( $_FILES["file"]["name"])). " has been uploaded.";
        } else {
            echo "Sorry, there was an error uploading your file.";
        }
    }
}
?>