The DownloadFile() function will initiate a download from the path that you provided. It works great with large files because it buffers the data, instead of putting the entire content of the file in memory and serving it from there.

1.  <?php
2.  function DownloadFile($File)
3.  {
4.      if (!is_file($File))
5.          {
6.                  die("404 File not found!");
7.          }
8.     
9.      $FileName = basename($File);
10.     $FileExt = strtolower(substr(strrchr($FileName,"."),1));
11.    
12.     switch($FileExt)
13.         {
14.         case "exe":
15.                         $ctype = "application/octet-stream";
16.                         break;
17.         case "zip":
18.                         $ctype = "application/zip";
19.                         break;
20.         case "mp3":
21.                         $ctype = "audio/mpeg";
22.                         break;
23.         case "mpg":
24.                         $ctype = "video/mpeg";
25.                         break;
26.         case "avi":
27.                         $ctype = "video/x-msvideo";
28.                         break;
29.                 case "wmv":
30.                         $ctype = "video/x-ms-wmv";
31.                         break;
32.         default:
33.                         $ctype="application/force-download";
34.     }
35.    
36.     header("Cache-Control:");
37.     header("Cache-Control: public");
38.    
39.     header("Content-Type: $ctype");
40.     if (strstr($_SERVER['HTTP_USER_AGENT'], "MSIE"))
41.         {
42.         // IE Bug workaround
43.         $IEFileName = preg_replace('/\./', '%2e', $FileName, substr_count($FileName, '.') - 1);
44.         header("Content-Disposition: attachment; filename=\"$IEFileName\"");
45.     }
46.         else
47.         {
48.         header("Content-Disposition: attachment; filename=\"$FileName\"");
49.     }
50.     header("Accept-Ranges: bytes");
51.    
52.     $FileSize = filesize($File);
53.     // If http_range is sent by browser (or download manager)
54.     if(isset($_SERVER['HTTP_RANGE']))
55.         {
56.         list($a, $Range) = explode("=",$_SERVER['HTTP_RANGE']);
57.         // If yes, download missing part
58.         str_replace($Range, "-", $Range);
59.         $FileSize2 = $FileSize-1;
60.         $new_length = $FileSize2-$Range;
61.         header("HTTP/1.1 206 Partial Content");
62.         header("Content-Length: $new_length");
63.         header("Content-Range: bytes $Range$FileSize2/$FileSize");
64.     }
65.         else
66.         {
67.         $FileSize2 = $FileSize-1;
68.         header("Content-Range: bytes 0-$FileSize2/$FileSize");
69.         header("Content-Length: ".$FileSize);
70.     }
71.     // Open the file
72.     $FP = fopen("$File","rb");
73.     // Seek to start of missing part
74.     fseek($FP,$Range);
75.     // Start buffered download
76.     while(!feof($FP))
77.         {
78.         // Reset time limit so there's no timeout
79.         set_time_limit(0);
80.         print(fread($FP,1024*8));
81.         flush();
82.         ob_flush();
83.     }
84.     fclose($FP);
85.     exit;
86. }
87. // How to use it
88. DownloadFile("F:\Geekpedia Exposed.wmv");
89. ?>