File upload and download technology in PHP

PHP is a programming language widely used in Web development. It is characterized by its simplicity, ease of learning, strong scalability, and short development cycle, so it is widely loved by developers. In web development, file uploading and downloading is a common requirement, and PHP provides some built-in functions and classes to help us implement these functions conveniently. This article will introduce file upload and download technology in PHP.

1. File upload technology

1. HTML form

In HTML, we can use the type attribute of the input tag to be "file" to create a The file upload form element is as follows:

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

It should be noted that the form needs to be submitted using the POST method, and the enctype attribute needs to be set to "multipart/form-data" so that the form can support file upload. At the same time, we need to create an input element with a name attribute of "fileToUpload". This attribute value will be used in the subsequent PHP code to obtain the uploaded file.

2. PHP processing

After the user submits the form to upload the file, we need to process it in the background. In PHP, we can use the $_FILES super global array to get the uploaded files.

<?php
$target_dir = "./uploads/";
$target_file = $target_dir . basename($_FILES["fileToUpload"]["name"]);
$uploadOk = 1;
$imageFileType = strtolower(pathinfo($target_file,PATHINFO_EXTENSION));

// 检查文件是否是图片
if(isset($_POST["submit"])) {
  $check = getimagesize($_FILES["fileToUpload"]["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["fileToUpload"]["size"] > 500000) {
  echo "Sorry, your file is too large.";
  $uploadOk = 0;
}

// 允许上传的文件类型
if($imageFileType != "jpg" && $imageFileType != "png" && $imageFileType != "jpeg"
&& $imageFileType != "gif" ) {
    echo "Sorry, only JPG, JPEG, PNG & GIF files are allowed.";
    $uploadOk = 0;
}

// 如果所有检查都通过，上传文件
if ($uploadOk == 0) {
  echo "Sorry, your file was not uploaded.";
} else {
  if (move_uploaded_file($_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.";
  }
}
?>

In the above code, we first define a directory $target_dir to store uploaded files, and then generate a target file path $target_file based on the file name uploaded by the user and $target_dir. Then, we performed a series of checks on the uploaded files, such as:

• Check whether the file is an image file
• Check whether the file already exists
• Check the file Whether the size is too large
• Check whether the uploaded file type is allowed

Finally, if all checks pass, we use the move_uploaded_file function to move the uploaded file from the tmp directory to $ The location specified by target_file.

2. File Download Technology

File downloading is generally divided into two ways. One is to download static files (such as pictures, text files, audio and video files, etc.), which is relatively simple. ;The other is to download dynamically generated files, which is relatively more troublesome.

1. Download static files

In PHP, we can download files through the header() function, for example:

$file = "./downloads/test.txt";

// 设置HTTP头信息以告诉浏览器下载文件
header("Content-disposition: attachment;filename=".basename($file));
header("Content-type: application/octet-stream");
header("Content-Length: ".filesize($file));
header("Pragma: no-cache");
header("Expires: 0");

// 将文件内容读取并输出给浏览器
readfile($file);

The above In the code, we first specify a file $file and use the header() function to set the HTTP header information. Among them, Content-disposition specifies the name of the downloaded file, Content-type specifies the file type, Content-Length specifies the file size, and Pragma and Expires are used to prevent the browser from caching the downloaded file.

Finally, we use the readfile() function to read the contents of the specified file and output it to the client browser, thus realizing the file download function.

2. Download dynamically generated files

For dynamically generated files, we need to save them to the server first, and then download them as static files.

$content = "Hello, world!";
$filename = "test.txt";
$file = "./downloads/".$filename;

// 将内容写入到文件中
$handle = fopen($file, "w");
fwrite($handle, $content);
fclose($handle);

// 返回下载文件给客户端
header("Content-disposition: attachment;filename=".$filename);
header("Content-type: application/octet-stream");
header("Content-Length: ".filesize($file));
header("Pragma: no-cache");
header("Expires: 0");
readfile($file);

// 下载完成后删除服务器端的文件
unlink($file);

In the above code, we first save the dynamically generated file $content to a file $filename on the server side, and then set the HTTP header information through the header() function to let the client browser download file. Finally, we use the unlink() function to delete the file on the server side to prevent the file from occupying server-side resources.

Summary

This article introduces the file upload and download technology in PHP. These functions are very practical for developing Web applications. When using PHP to upload and download files, you need to pay attention to some security issues, such as checking file type, size, file name, etc., to avoid attacks by malicious users. At the same time, we should also try our best to optimize the performance of file upload and download, reduce the waste of resources on the server and client, and improve the user experience.

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