PHP Development Guide: How to implement file upload and download functions

PHP Development Guide: How to implement file upload and download functions

Introduction:
In modern web applications, file upload and download are one of the common functions one. Whether it's uploading a user's profile picture or downloading a provided document, file handling is an inevitable part of our PHP development. This article will introduce how to use PHP to implement file upload and download functions, and provide code examples so that readers can better understand and apply it.

1. Implementation of the file upload function
File upload refers to an operation of transferring files on the local computer to the server. Here are some key steps to implement file upload functionality:

1. Create an HTML form. First, we need to create an HTML form on the front end so that users can select and upload files. In the HTML form, we need to specify the enctype attribute of file upload as "multipart/form-data".

Sample code:

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

2. Create a file upload processing script. Next, we need to create a PHP script to handle the file upload. In the script, we can use the $_FILES array to access the uploaded files. Among them, $_FILES'fileToUpload' represents the original name of the uploaded file, and $_FILES'fileToUpload' represents the temporary storage path of the file on the server.

Sample code:

<?php
   $targetDir = "uploads/";
   $targetFile = $targetDir . basename($_FILES["fileToUpload"]["name"]);
   $uploadOk = 1;
   $fileType = strtolower(pathinfo($targetFile,PATHINFO_EXTENSION));
   
   // Check if file already exists
   if (file_exists($targetFile)) {
       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($fileType != "jpg" && $fileType != "png" && $fileType != "jpeg"
   && $fileType != "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"], $targetFile)) {
           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 move the temporarily uploaded file to the specified target directory through the move_uploaded_file() function.

2. Implementation of file download function
File download refers to an operation of transferring files from the server to the local computer. The following are some key steps to implement the file download function:

1. Set the HTTP header. First, we need to set some HTTP headers to tell the browser that the file is to be downloaded. We can use the header() function to set Content-type and Content-Disposition.

Sample code:

<?php
   header("Content-type: application/octet-stream");
   header("Content-Disposition: attachment; filename=example.pdf");
   readfile("example.pdf");
?>

In the above code, we set the Content-type to application/octet-stream, which means telling the browser to download the file in the form of a binary stream. Content-Disposition is set to attachment, and the file name is specified as example.pdf.

2. Set the file path and name. The readfile() function is used to read a file and send it to the output buffer. In the readfile() function, we need to specify the path and name of the file to be downloaded.

The above is the key code to implement the file download function.

Conclusion:
Through this article, we learned how to use PHP to implement file upload and download functions. To upload files, you need to specify the enctype attribute as "multipart/form-data" in the HTML form, and access the uploaded file information through the $_FILES array. File downloading requires setting the Content-type and Content-Disposition headers, and using the readfile() function to send the file to the output buffer. Through the above sample code, we can better understand and apply it to actual projects.

To sum up, file uploading and downloading are very common functions in modern web applications, and they are also a part that needs to be mastered in PHP development. I hope this article can be helpful to readers and enable file upload and download functions in practical applications.

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