The most important thing to remember when sending headers from
PHP is that all of the headers must be sent prior to any Web content.
If even a single character of the Web page is sent prior to sending
the header, the header information will be considered text within
the Web page and not header information. This will prevent your file
from being downloaded. The easiest way to avoid this problem is to
ensure that the first characters on the first line of the PHP script are
the opening PHP tag (<?php).
PHP uses the header() function to return header information to the
client Web browser. The header() function takes a single parameter,
which is a text string containing the name of the header field followed
by a colon, a space, and the data to associate with the header. The
headers listed in Table 5-7 are far from comprehensive, but they are
the only ones you need to download a file.
For example, the following headers tell the Web browser that a file
named info.doc is 5000 bytes long, is encoded using base64 encoding,
and is a file being downloaded:
CHAPTER
5
Working
with Files and Directories
header("Content-Description:
File Transfer");
header("Content-Type:
application/force-download");
header("Content-Disposition:
attachment;
filename=\"info.doc\"");
header("Content-Transfer-Encoding: base64");
header("Content-Length: 5000");
254
The “Content-Type” header can be used in two ways. If you want the
downloaded file to appear in the client’s Web browser as if it were a
normal file, use the MIME type for the file. For example, if you are
downloading a JPEG image, the MIME type would be “image/jpeg”.
The Web browser will display the image. If you want the file to be
saved to the user’s hard drive instead of being opened in the Web
browser, use a MIME type of “application/force-download”, which
instructs the Web browser to open a “save file” dialog box and write
the file to disk.
The third step, once the headers have been sent, is to send the file
itself. The PHP readfile() function reads a file from disk and sends
it directly to the Web browser. The only required parameter for the
readfile() function is a string containing the path and filename to
the file being sent; on success, readfile() returns the number of bytes
sent; on failure, readfile() returns FALSE.
readfile("/usr/uploads/info.doc");
You are finished when the headers have been sent and the readfile()
function has sent the file’s contents. Do not send any XHTML data,
or it will become part of the downloaded file information. If, however,
the headers were not sent and the readfile() function was not called,
the PHP script can create a Web page explaining that the file could
not be downloaded and why. Therefore, your code should ensure that
the headers were not sent and the readfile() function was not called
prior to sending the Web page output.
To create a PHP downloader for the files subdirectory:
1.
2.
Create a new document in your text editor.
Add the following script section to the document body. Be sure
that there is nothing in the file before the opening PHP tag:
<?php
?>
3.
Add the following code to the script section to check if the
requested file exists and is readable:
$Dir = "files";
if (isset($_GET['filename'])) {
$FileToGet = $Dir . "/" . stripslashes
($_GET['filename']);
Uploading
and Downloading Files
if
(is_readable($FileToGet)) {
}
else
{
$ErrorMsg
= "Cannot read \"$FileToGet\"";
$ShowErrorPage = TRUE;
}
}
else {
$ErrorMsg = "No filename specified";
$ShowErrorPage = TRUE;
}
if ($ShowErrorPage) {
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4.
Add the following code in the if section of the inner
if...else statement for the is_readable() test to download
the file:
header("Content-Description: File Transfer");
header("Content-Type: application/force-download");
header("Content-Disposition: attachment;
filename=\"" . $_GET['filename'] . "\"");
header("Content-Transfer-Encoding: base64");
header("Content-Length: " . filesize($FileToGet));
readfile($FileToGet);
$ShowErrorPage = FALSE;
5.
Add the following code immediately after the closing PHP tag
to show the error page. Note the use of advanced escaping to
display the Web page output.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0
Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>File Download Error</title>
<meta http-equiv="content-type" content="text/html;
charset=iso-8859-1" />
</head>
<body>
<p>There was an error downloading "<?php echo
htmlentities($_GET['filename']); ?>"</p>
<p><?php echo htmlentities($ErrorMsg); ?></p>
</body>
</html>
<?php
}
?>
6.
Save the document as FileDownloader.php in the Chapter
directory for Chapter 5 and upload the file to the server.
Reopen ViewFiles.php. Replace the line that reads:
echo "<a href=\"$FullEntryName\">" .
htmlentities($Entry). "</a>\n";
7.
CHAPTER
5
Working
with Files and Directories
with
a line that reads:
echo
"<a
href=\"FileDownloader.php?filename=$Entry\">"
.
htmlentities($Entry). "</a>\n";
8.
256
9.
Save ViewFiles.php and upload the file to the Web server.
Open the ViewFiles.php file in your Web browser by entering
the following URL: http://<yourserver>/PHP_Projects/Chap-
ter.05/Chapter/ViewFiles.php. Click one of the highlighted
filenames. Your Web browser should display a “save file”
dialog box like the one shown in Figure 5-8. Save the file and
verify that it downloaded correctly.
Figure 5-8
The “save file” dialog box for polarbear.gif
10. Close your Web browser window.
Short Quiz
1.
What type of form input element is used to choose the file to
upload?
What hidden form input element restricts the size of the
uploaded file?
What is the name of the autoglobal array that contains the
uploaded file information?
2.
3.
