如何在 PHP 中实现文件上传功能？

在 PHP 中实现文件上传功能需要使用表单和 PHP 的文件上传函数。具体步骤如下：

1. 创建一个 HTML 表单，设置 enctype 属性为 "multipart/form-data"，并添加一个文件选择框：

   <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>

2. 在 PHP 文件中，使用 $_FILES 变量获取上传的文件信息：

   $target_dir = "uploads/";
   $target_file = $target_dir . basename($_FILES["fileToUpload"]["name"]);
   $uploadOk = 1;

3. 检查文件是否已经存在：

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

4. 检查文件大小是否符合要求：

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

5. 检查文件类型是否符合要求：

   $allowedFileType = array("jpg", "jpeg", "png", "gif");
   $fileType = strtolower(pathinfo($target_file,PATHINFO_EXTENSION));
   if(!in_array($fileType, $allowedFileType)) {
    echo "Sorry, only JPG, JPEG, PNG, GIF files are allowed.";
    $uploadOk = 0;
   }

6. 检查文件是否上传成功：

   if ($uploadOk == 0) {
    echo "Sorry, your file was not uploaded.";
   } else {
    if (move_uploaded_file($_FILES["fileToUpload"]["tmp_name"], $target_file)) {
        echo "The file ". basename( $_FILES["fileToUpload"]["name"]). " has been uploaded.";
    } else {
        echo "Sorry, there was an error uploading your file.";
    }
   }