File Uploads with PHP

What do pictures in an online photo album, email attachments in a web-based mail client, and data files submitted to an online application for batch processing all have in common? They all rely on the ability to upload files across the Internet from the user’s web browser. Indeed, uploading files is an important feature of many of the sites and web-based applications we use on a daily basis. In this post, I show you how to add support for file uploads to your site using PHP.

Requirements

Handling file uploads isn’t difficult, but there are a handful of small details that must be correct or else the upload will fail. First, you need to ensure PHP is configured to allow uploads. Check your php.ini file and verify the file_uploads directive is set On.

file_uploads = On

Uploaded files are first stored in a temporary directory (don’t worry… your PHP script can move the files to a more permanent location afterward). By default, the initial location is the system’s default temporary directory. You can specify a different directory using the upload_tmp_dir directive in php.ini. Regardless, you should verify the PHP process has the proper privileges to write to whichever directory is used.

upload_tmp_dir = "/tmp"

tboronczyk@zarkov:~$ ls -l / | grep tmp
drwxrwxrwt  13 root root 40960 2011-08-31 00:50 tmp

After you’re certain the configuration allows the server to accept uploaded files, you can focus your attention on the HTML details. As with most other server-side interactions from HTML, uploading files make use of forms. It is imperative that your <form> element uses the POST method and has an enctype attribute set to multipart/form-data.

1 <form action="upload.php" method="post" enctype="multipart/form-data">

Scripting the Upload Process

You can probably guess the workflow file uploads go through based on your own experiences and the requirement checks I’ve just mentioned.

  • A visitor views an HTML page with a form specifically written to support file uploads
  • The visitor provides the file he wants to upload and submits the form
  • The browser encodes the file and sends it as part of the POST request it makes to the server
  • PHP receives the form submission, decodes the file and saves it in a temporary location on the server
  • The PHP script responsible for handling the form post verifies the file and processes it in some manner, often moving it from its temporary location to a more permanent home

Adding support for file uploads requires you to create an HTML form to be presented to the user and a PHP script to take care of the uploaded file on the server.

HTML

HTML forms provide the interface through which a user initiates a file upload. Remember, the <form> element must have its method attribute set to post and its enctype attribute set to multipart/form-data. A file <input> element provides the a field used to specify the file that will be upload. Like any other form element, it is important you provide a name attribute so you can reference it in the PHP script that processes the form.

Here’s what markup for a basic file upload form looks like:

1 <form action="upload.php" method="post" enctype="multipart/form-data">
2  <input type="file" name="myFile">
3  <br>
4  <input type="submit" value="Upload">
5 </form>

It’s worth noting that different browsers will render the file field differently. IE, Firefox, and Opera display it as a text field with a button next to it labeled “Browse” or “Choose.” Safari renders it just as button labeled “Choose File.” This isn’t a problem most of the time since users are accustomed to how the field renders in their browser of choice and know how to use it. Occasionally, however, you will be faced with a client or designer who is adamant on presenting it a certain way. The amount of CSS and JavaScript that can be applied to a file field is extremely limited because of security reasons imposed by the browsers. Styling the file field can be difficult. If appearance is important for you, I recommend you check out Peter-Paul Koch’s Styling an input type=”file”.

PHP

Information about the file upload is made available with the multidimensional $_FILES array. This array is indexed by the names assigned to the file fields in the HTML form, just as how $_GET and $_POST are indexed. Each file’s array then contains the following indexes:

  • $_FILES["myFile"]["name"] stores the original filename from the client
  • $_FILES["myFile"]["type"] stores the file’s mime-type
  • $_FILES["myFile"]["size"] stores the file’s size (in bytes)
  • $_FILES["myFile"]["tmp_name"] stores the name of the temporary file
  • $_FILES[“myFile”][“error”] stores any error code resulting from the transfer

The move_uploaded_file() function moves an uploaded file from its temporary to permanent location. You should always use move_uploaded_file() over functions like copy() and rename() for this purpose because it performs additional checks to ensure the file was indeed uploaded by the HTTP POST request.

If you plan on saving a file with the original filename provided by the user, it’s a good idea to make sure it’s safe to do so. The filename should not contain any characters that can affect the destination path, such as a slash. The name shouldn’t cause the file to overwrite an existing file with the same name, either (unless that’s what your application is designed to do). I ensure a safe filename by replacing any characters with an underscore that aren’t a letter, number, or a member of a very restricted set of punctuation, and then append an incrementing number when a file by that name already exists.

Here’s what receiving and processing a file upload with PHP looks like:

01 <?php
02 define("UPLOAD_DIR", "/srv/www/uploads/");
03
04 if (!empty($_FILES["myFile"])) {
05     $myFile = $_FILES["myFile"];
06
07     if ($myFile["error"] !== UPLOAD_ERR_OK) {
08         echo "<p>An error occurred.</p>";
09         exit;
10     }
11
12     // ensure a safe filename
13     $name = preg_replace("/[^A-Z0-9._-]/i", "_", $myFile["name"]);
14
15     // don't overwrite an existing file
16     $i = 0;
17     $parts = pathinfo($name);
18     while (file_exists(UPLOAD_DIR . $name)) {
19         $i++;
20         $name = $parts["filename"] . "-" . $i . "." . $parts["extension"];
21     }
22
23     // preserve file from temporary directory
24     $success = move_uploaded_file($myFile["tmp_name"],
25         UPLOAD_DIR . $name);
26     if (!$success) {
27         echo "<p>Unable to save file.</p>";
28         exit;
29     }
30
31     // set proper permissions on the new file
32     chmod(UPLOAD_DIR . $name, 0644);
33 }

The code first makes sure the file uploaded without any errors. It then determines a safe filename as I just described, and then moves the file to its final directory using move_uploaded_file(). Finally, there is a call to chmod() to make sure sane access permissions are set on the new file.

Security Considerations

Most of us wouldn’t let complete strangers store random files on our personal computers, and yet that is exactly what you are doing when you allow file uploads in our application. You may intend for a user to upload a picture of himself for a profile page, but what if he tries to upload a specially-crafted, virus-laden executable instead? I’d like to share a few steps that you can take to minimize the security risks inherent in allowing file uploads.

One is to verify the type of the uploaded file is what it should be. Relying on either the value of $_FILES["myFile"]["type"] or on the filename’s extension isn’t secure because both can easily be spoofed. Rather, use a function like exif_imagetype() to examine the contents of the file and determine if it is indeed a GIF, JPEG, or one of several other supported image formats. If exif_imagetype() isn’t available (the function requires the Exif extension to be enabled), then you can use getimagesize(). The array returned by getimagesize() will contain the image type if it is recognized.

1 <?php
2 // verify the file is a GIF, JPEG, or PNG
3 $fileType = exif_imagetype($_FILES["myFile"]["tmp_name"]);
4 $allowed = array(IMAGETYPE_GIF, IMAGETYPE_JPEG, IMAGETYPE_PNG);
5 if (!in_array($fileType, $allowed)) {
6     // file type is not permitted
7     ...

For non-image files, you can use exec() to invoking the unix file utility. file determines a file’s type by looking for known binary signatures in expected locations.

1 <?php
2 // verify the file is a PDF
3 $mime = "application/pdf; charset=binary";
4 exec("file -bi " . $_FILES["myFile"]["tmp_name"], $out);
5 if ($out[0] != $mime) {
6     // file is not a PDF
7     ...

Another step you can take is to impose hard limits on the total size of the POST request and the number of files that can be uploaded. To do so, specify an appropriate value for the upload_max_size, post_max_size, and max_file_uploads directives in php.ini. The upload_max_size directive specifies the maximum size a file upload can be. In addition to the size of the upload, you can limit the size of the entire POST request with the post_max_size directive. max_file_uploads is a newer directive (added in version 5.2.12) which limits the number of file uploads. These three directives help protect your site against attacks that try to disrupt its availability by causing heavy network traffic or system load.

post_max_size = 8M
upload_max_size = 2M
max_file_uploads = 20

A third step you can take to minimize your risk is to scan uploaded files with a virus scanner. This is vitally important in this day and age of widespread viruses and malware, especially if your site later makes uploaded files available for download by other individuals, such as with attachments in a web-based email client or a (legal) file-sharing site. There is a PHP extension that provides access to ClamAV, but of course you can invoke ClamAV’s command-line utility in much the same way I demonstrated for file.

1 <?php
2 exec("clamscan --stdout " . $_FILES["myFile"]["tmp_name"], $out, $return);
3 if ($return) {
4     // file is infected
5     ...

Summary

You’ve learned how easy it is to support file uploads with your site or web-based application. For the upload to succeed, the HTML form must be submitted via a multipart/form-data encoded POST request, and PHP must permit the transfer as specified using the file_uploads directive. After the file is transferred, the script responsible for handling the upload uses the information found in the $_FILES array to move the file from the temporary directory to the desired location. I also shared some extra precautions that you can take to protect yourself and your users from some of the risks associated with allowing file uploads. You saw how you can ensure the filename is safe, verify the file type, impose hard limits on upload traffic, and scan for viruses.

For those who may be interested, supplemental code for this article is available under PHPMaster’s GitHub account. You can view, download, or clone the repository and play with the code to get a better understanding of how the process of uploading files works.

Referenced Link ╠►

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By Rz Rasel Posted in Php

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