readfile

Just a note for those who face problems on names containing spaces (e.g. "test test.pdf").
In the examples (99% of the time) you can find
header('Content-Disposition: attachment; filename='.basename($file));
but the correct way to set the filename is quoting it (double quote):
header('Content-Disposition: attachment; filename="'.basename($file).'"' );
Some browsers may work without quotation, but for sure not Firefox and as Mozilla explains, the quotation of the filename in the content-disposition is according to the RFC
http://kb.mozillazine.org/Filenames_with_spaces_are_truncated_upon_download 

if you need to limit download rate, use this code 
<?php
$local_file = 'file.zip';
$download_file = 'name.zip';
// set the download rate limit (=> 20,5 kb/s)
$download_rate = 20.5;
if(file_exists($local_file) && is_file($local_file))
{
 header('Cache-control: private');
 header('Content-Type: application/octet-stream');
 header('Content-Length: '.filesize($local_file));
 header('Content-Disposition: filename='.$download_file);
 flush();
 $file = fopen($local_file, "r");
 while(!feof($file))
 {
 // send the current file part to the browser
 print fread($file, round($download_rate * 1024));
 // flush the content to the browser
 flush();
 // sleep one second
 sleep(1);
 }
 fclose($file);}
else {
 die('Error: The file '.$local_file.' does not exist!');
}
?>

Always using MIME-Type 'application/octet-stream' is not optimal. Most if not all browsers will simply download files with that type.
If you use proper MIME types (and inline Content-Disposition), browsers will have better default actions for some of them. Eg. in case of images, browsers will display them, which is probably what you'd want.
To deliver the file with the proper MIME type, the easiest way is to use:
header('Content-Type: ' . mime_content_type($file)); 
header('Content-Disposition: inline; filename="'.basename($file).'"');

To avoid the risk of choosing themselves which files to download by messing with the request and doing things like inserting "../" into the "filename", simply remember that URLs are not file paths, and there's no reason why the mapping between them has to be so literal as "download.php?file=thingy.mpg" resulting in the download of the file "thingy.mpg".
It's your script and you have full control over how it maps file requests to file names, and which requests retrieve which files.
But even then, as ever, never trust ANYTHING in the request. Basic first-day-at-school security principle, that.

To anyone that's had problems with Readfile() reading large files into memory the problem is not Readfile() itself, it's because you have output buffering on. Just turn off output buffering immediately before the call to Readfile(). Use something like ob_end_flush().

A note on the smartReadFile function from gaosipov:
Change the indexes on the preg_match matches to:
 
 $begin = intval($matches[1]);
 if( !empty($matches[2]) ) {
 $end = intval($matches[2]);
 }
Otherwise the $begin would be set to the entire section matched and the $end to what should be the begin.
See preg_match for more details on this.

My script working correctly on IE6 and Firefox 2 with any typ e of files (I hope :))
function DownloadFile($file) { // $file = include path 
 if(file_exists($file)) {
 header('Content-Description: File Transfer');
 header('Content-Type: application/octet-stream');
 header('Content-Disposition: attachment; filename='.basename($file));
 header('Content-Transfer-Encoding: binary');
 header('Expires: 0');
 header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
 header('Pragma: public');
 header('Content-Length: ' . filesize($file));
 ob_clean();
 flush();
 readfile($file);
 exit;
 }
 }
Run on Apache 2 (WIN32) PHP5

regarding php5:
i found out that there is already a disscussion @php-dev about readfile() and fpassthru() where only exactly 2 MB will be delivered.
so you may use this on php5 to get lager files
<?php
function readfile_chunked($filename,$retbytes=true) {
 $chunksize = 1*(1024*1024); // how many bytes per chunk
 $buffer = '';
 $cnt =0;
 // $handle = fopen($filename, 'rb');
 $handle = fopen($filename, 'rb');
 if ($handle === false) {
 return false;
 }
 while (!feof($handle)) {
 $buffer = fread($handle, $chunksize);
 echo $buffer;
 if ($retbytes) {
 $cnt += strlen($buffer);
 }
 }
 $status = fclose($handle);
 if ($retbytes && $status) {
 return $cnt; // return num. bytes delivered like readfile() does.
 } 
 return $status;
} 
?>

For anyone having the problem of your html page being outputted in the downloaded file: call the functions ob_clean() and flush() before readfile()

Send file with HTTPRange support (partial download):
<?php
function smartReadFile($location, $filename, $mimeType='application/octet-stream')
{ if(!file_exists($location))
 { header ("HTTP/1.0 404 Not Found");
 return;
 }
 
 $size=filesize($location);
 $time=date('r',filemtime($location));
 
 $fm=@fopen($location,'rb');
 if(!$fm)
 { header ("HTTP/1.0 505 Internal server error");
 return;
 }
 
 $begin=0;
 $end=$size;
 
 if(isset($_SERVER['HTTP_RANGE']))
 { if(preg_match('/bytes=\h*(\d+)-(\d*)[\D.*]?/i', $_SERVER['HTTP_RANGE'], $matches))
 { $begin=intval($matches[0]);
 if(!empty($matches[1]))
 $end=intval($matches[1]);
 }
 }
 
 if($begin>0||$end<$size)
 header('HTTP/1.0 206 Partial Content');
 else
 header('HTTP/1.0 200 OK'); 
 
 header("Content-Type: $mimeType"); 
 header('Cache-Control: public, must-revalidate, max-age=0');
 header('Pragma: no-cache'); 
 header('Accept-Ranges: bytes');
 header('Content-Length:'.($end-$begin));
 header("Content-Range: bytes $begin-$end/$size");
 header("Content-Disposition: inline; filename=$filename");
 header("Content-Transfer-Encoding: binary\n");
 header("Last-Modified: $time");
 header('Connection: close'); 
 
 $cur=$begin;
 fseek($fm,$begin,0);
 while(!feof($fm)&&$cur<$end&&(connection_status()==0))
 { print fread($fm,min(1024*16,$end-$cur));
 $cur+=1024*16;
 }
}
?>

Usage:
<?php
smartReadFile("/tmp/filename","myfile.mp3","audio/mpeg")
?>

It can be slow for big files to read by fread, but this is a single way to read file in strict bounds. You can modify this and add fpassthru instead of fread and while, but it sends all data from begin --- it would be not fruitful if request is bytes from 100 to 200 from 100mb file.

If you are lucky enough to not be on shared hosting and have apache, look at installing mod_xsendfile.
This was the only way I found to both protect and transfer very large files with PHP (gigabytes). 
It's also proved to be much faster for basically any file.
Available directives have changed since the other note on this and XSendFileAllowAbove was replaced with XSendFilePath to allow more control over access to files outside of webroot.
Download the source.
Install with: apxs -cia mod_xsendfile.c
Add the appropriate configuration directives to your .htaccess or httpd.conf files:
# Turn it on
XSendFile on
# Whitelist a target directory.
XSendFilePath /tmp/blah
Then to use it in your script:
<?php
$file = '/tmp/blah/foo.iso';
$download_name = basename($file);
if (file_exists($file)) {
 header('Content-Type: application/octet-stream');
 header('Content-Disposition: attachment; filename='.$download_name);
 header('X-Sendfile: '.$file);
 exit;
}
?>

In response to flowbee@gmail.com --
When using the readfile_chunked function noted here with files larger than 10MB or so I am still having memory errors. It's because the writers have left out the all important flush() after each read. So this is the proper chunked readfile (which isn't really readfile at all, and should probably be crossposted to passthru(), fopen(), and popen() just so browsers can find this information):
<?php
function readfile_chunked($filename,$retbytes=true) {
 $chunksize = 1*(1024*1024); // how many bytes per chunk
 $buffer = '';
 $cnt =0;
 // $handle = fopen($filename, 'rb');
 $handle = fopen($filename, 'rb');
 if ($handle === false) {
 return false;
 }
 while (!feof($handle)) {
 $buffer = fread($handle, $chunksize);
 echo $buffer;
 ob_flush();
 flush();
 if ($retbytes) {
 $cnt += strlen($buffer);
 }
 }
 $status = fclose($handle);
 if ($retbytes && $status) {
 return $cnt; // return num. bytes delivered like readfile() does.
 }
 return $status;
}
?>

All I've added is a flush(); after the echo line. Be sure to include this!