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Chapter 19. Handling file uploads

Table of Contents
POST method uploads
Common Pitfalls
Uploading multiple files
PUT method support

POST method uploads

PHP is capable of receiving file uploads from any RFC-1867 compliant browser (which includes Netscape Navigator 3 or later, Microsoft Internet Explorer 3 with a patch from Microsoft, or later without a patch). This feature lets people upload both text and binary files. With PHP's authentication and file manipulation functions, you have full control over who is allowed to upload and what is to be done with the file once it has been uploaded.

Note that PHP also supports PUT-method file uploads as used by Netscape Composer and W3C's Amaya clients. See the PUT Method Support for more details.

A file upload screen can be built by creating a special form which looks something like this:

Example 19-1. File Upload Form

<form enctype="multipart/form-data" action="_URL_" method="post">
<input type="hidden" name="MAX_FILE_SIZE" value="1000">
Send this file: <input name="userfile" type="file">
<input type="submit" value="Send File">
</form>
The _URL_ should point to a PHP file. The MAX_FILE_SIZE hidden field must precede the file input field and its value is the maximum filesize accepted. The value is in bytes.

Warning

The MAX_FILE_SIZE is advisory to the browser. It is easy to circumvent this maximum. So don't count on it that the browser obeys you wish! The PHP-settings for maximum-size, however, cannot be fooled.

In PHP, the following variables will be defined within the destination script upon a successful upload, assuming that register_globals is turned on in php.ini. If track_vars is turned on, they will also be available in PHP within the global array $HTTP_POST_VARS. Note that the following variable names assume the use of the file upload name 'userfile', as used in the example above:

Note that the "$userfile" part of the above variables is whatever the name of the INPUT field of TYPE=file is in the upload form. In the above upload form example, we chose to call it "userfile"

In PHP 4, the behaviour is slightly different, in that the new global array $HTTP_POST_FILES is provided to contain the uploaded file information. This is still only available if track_vars is turned on, but track_vars is always turned on in versions of PHP after PHP 4.0.2.

The contents of $HTTP_POST_FILES are as follows. Note that this assumes the use of the file upload name 'userfile', as used in the example above:

$HTTP_POST_FILES['userfile']['name']

The original name of the file on the client machine.

$HTTP_POST_FILES['userfile']['type']

The mime type of the file, if the browser provided this information. An example would be "image/gif".

$HTTP_POST_FILES['userfile']['size']

The size, in bytes, of the uploaded file.

$HTTP_POST_FILES['userfile']['tmp_name']

The temporary filename of the file in which the uploaded file was stored on the server.

Files will by default be stored in the server's default temporary directory, unless another location has been given with the upload_tmp_dir directive in php.ini. The server's default directory can be changed by setting the environment variable TMPDIR in the environment in which PHP runs. Setting it using putenv() from within a PHP script will not work. This environment variable can also be used to make sure that other operations are working on uploaded files, as well.

Example 19-2. Validating file uploads

The following examples are for versions of PHP 3 greater than 3.0.16, and versions of PHP 4 greater than 4.0.2. See the function entries for is_uploaded_file() and move_uploaded_file(). 

<?php 
if (is_uploaded_file($userfile)) {
    copy($userfile, "/place/to/put/uploaded/file");
} else {
    echo "Possible file upload attack: filename '$userfile'.";
}
/* ...or... */
move_uploaded_file($userfile, "/place/to/put/uploaded/file");
?>

For earlier versions of PHP, you'll need to do something like the following.

Note: This will not work in versions of PHP 4 after 4.0.2. It depends on internal functionality of PHP which changed after that version. 

<?php
/* Userland test for uploaded file. */ 
function is_uploaded_file($filename) {
    if (!$tmp_file = get_cfg_var('upload_tmp_dir')) {
        $tmp_file = dirname(tempnam('', ''));
    }
    $tmp_file .= '/' . basename($filename);
    /* User might have trailing slash in php.ini... */
    return (ereg_replace('/+', '/', $tmp_file) == $filename);
}

if (is_uploaded_file($userfile)) {
    copy($userfile, "/place/to/put/uploaded/file");
} else {
    echo "Possible file upload attack: filename '$userfile'.";
}
?>

The PHP script which receives the uploaded file should implement whatever logic is necessary for determining what should be done with the uploaded file. You can for example use the $file_size variable to throw away any files that are either too small or too big. You could use the $file_type variable to throw away any files that didn't match a certain type criteria. Whatever the logic, you should either delete the file from the temporary directory or move it elsewhere.

The file will be deleted from the temporary directory at the end of the request if it has not been moved away or renamed.

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