Detailed explanation of file upload and download techniques in PHP

PHP is a very popular server-side programming language that is widely used in website development. Among them, file uploading and downloading are one of the commonly used functions of websites, and PHP provides a wealth of functions and techniques to implement these functions. In this article, we will introduce in detail the file upload and download techniques in PHP so that you can develop your website more efficiently.

1. File upload

File upload refers to sending files from the local computer to the remote server. After uploading the files, we can store, process and display these files. operate. In PHP, we use the $_FILES superglobal variable to handle files uploaded by HTTP POST. The following is a basic file upload example:

<form enctype="multipart/form-data" method="POST" action="upload.php">
  <input type="file" name="file" />
  <input type="submit" value="上传" />
</form>

<?php
if ($_SERVER["REQUEST_METHOD"] == "POST") {
  $target_dir = "uploads/";
  $target_file = $target_dir . basename($_FILES["file"]["name"]);
  move_uploaded_file($_FILES["file"]["tmp_name"], $target_file);
  echo "文件上传成功！";
}
?>

In the above example, we use a ff9c23ada1bcecdd1a0fb5d5a0f18437 tag to create a file upload form, and specify the target address for form submission as upload. php. When the user selects the uploaded file and clicks the upload button, the form data will be submitted to upload.php via POST.

In upload.php, we first checked whether the value of $_SERVER["REQUEST_METHOD"] is POST to confirm whether the submission method is correct. Next, we specify a target directory $target_dir to save the uploaded file, and concatenate the uploaded file name and directory name to form a complete target file path $target_file. Finally, we used the move_uploaded_file() function to move the temporary file to the target directory, thereby completing the file upload operation.

In the process of file upload, in order to ensure the security of the system, we also need to perform various verification, filtering and processing of the uploaded files to prevent potential attacks and vulnerabilities. For example, we can limit and check uploaded file types, sizes, names, and contents, and we also need to deal with possible upload errors and exceptions.

2. File download

File download refers to downloading files from the remote server to the local computer. After downloading the files, we can save, print, analyze and other operations on these files. In PHP, we can use the header() function to set the HTTP response header information to implement the file download function. The following is a basic file download example:

<?php
$file = "example.pdf";
header("Content-Disposition: attachment; filename=" . basename($file));
header("Content-Type: application/pdf");
header("Content-Length: " . filesize($file));
readfile($file);
?>

In the above example, we first specify the file name example.pdf to be downloaded, and then use the header() function to set the response header information, including Content- Disposition (inline display or attachment download), Content-Type (MIME type) and Content-Length (file size), etc. Finally, we use the readfile() function to output the file content to implement the file download operation.

During the process of file downloading, in order to ensure system security and file integrity, we also need to consider possible file damage, midway stop, permission issues, download speed and other issues to improve the download experience. and reduce user dissatisfaction.

Summary:

This article introduces the file upload and download skills in PHP, hoping to help readers better understand and master these important website development functions. In actual development, we need to flexibly apply these techniques and functions according to specific needs and situations to develop safe, efficient, and easy-to-use website applications.

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