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(PHP 4, PHP 5)
symlink — Erzeugt einen symbolischen Link
symlink() erzeugt einen symbolischen Link auf das existierende Ziel (target) mit dem angegebenen Namen (link).
Ziel des Links.
Der Linkname.
Gibt bei Erfolg TRUE zurück. Im Fehlerfall wird FALSE zurückgegeben.
| Version | Beschreibung |
|---|---|
| 5.3.0 | Diese Funktion ist nun verfügbar auf Windows-Plattformen (Vista, Server 2008 oder neuer). |
Beispiel #1 Einen symbolischen Link erzeugen
<?php
$target = 'uploads.php';
$link = 'uploads';
symlink($target, $link);
echo readlink($link);
?>
Hinweis: Nur unter Windows: Diese Funktion wird nur funktionieren, falls das System, unter dem PHP ausgeführt wird, Windows Vista, Windows Server 2008 oder neuer ist. Frühere Windows-Versionen unterstützen keine symbolischen Links.
Symlinks on windows are created by Symlink() which accept only absolute paths but not relative paths .relative paths on windows are not supported for symlinks
Keep in mind when using a shared filesystem such as NFS, that you probably don't want to create a symbolic link with absolute paths e.g.
On Server1 you are running a PHP script that needs to create a symbolic link called widget2 which links to widget1 inside an NFS share mounted on your localhost at /mnt/nfs/widgets
On Server2 this same NFS share is mounted under /usr/local/widgets
If you used absolute paths on Server1, then Server2 would try to reference /mnt/nfs/widgets/widget1 which it won't be able to find...
You need to cd into the directory first, then create the link - that way the link will be relative. Unless you run the PHP script from the same directory where you'll be creating the symbolic links, then you can't use symlink(). Use exec() instead as follows:
<?php
exec("cd <nfs mount path>; ln -s <target> <link_name>");
?>
This function sometimes just fails to work for no appearent reason, returning FALSE and not creating a symlink. Doing the same with exec('ln -s source dest') works perfectly. I recommend using the latter.
Um, duh... that's all I gotta say about by previous note. Please delete it. :)
Windows Vista has its own symlink program now (mklink). Hopefully future versions of PHP for Windows will have this function put it?
Anyway, this will work on Vista (assuming your PHP user has the proper permissions):
<?php
define('SYMLINK_FILE', 0);
define('SYMLINK_DIR', 1);
define('SYMLINK_JUNCTION', 2);
function symlink ($target, $link, $flag = SYMLINK_FILE) {
switch ($flag) {
case SYMLINK_DIR: $pswitch = '/d'; break;
case SYMLINK_JUNCTION: $pswitch = '/j'; break;
case SYMLINK_FILE:
default: $pswitch = ''; break;
}
// Change / to \ because it will break otherwise.
$target = str_replace('/', '\\', $target);
$link = str_replace('/', '\\', $link);
return exec('mklink ' . $pswitch . ' "' . $link . '" "' . $target . '"');
}
?>
The one difference to using symlinks for controlled file access vs. readfile() is that the HTTP server will handle content-type of the symlink automatically.
If you always want it to be downloaded, this can be a negative point. However, if you want a file of non-predefined type to be viewable in the browser, this can be a real asset.
Of course, you can use fileinfo/mime-magic to do that, but those require a module which isn't always available on shared hosting.
<?
/**
* A function to emulate symbolic links on Windows.
* Uses the junction utility available at:
* http://www.sysinternals.com
* Note that this will only work on NTFS volumes.
*
* The syntax of the junction utility is:
* junction <junction directory> <junction target>
*
* Note that the parameter order of the Junction command
* is the reverse of the symlink function!
*
* @param string $target
* @param string $link
*/
function _symlink( $target, $link ) {
if ($_SERVER['WINDIR'] || $_SERVER['windir']) {
exec('junction "' . $link . '" "' . $target . '"');
} else {
symlink($target,$link);
}
}
function _unlink($link ) {
if ($_SERVER['WINDIR'] || $_SERVER['windir']) {
exec('junction -d "' . $link . '"');
} else {
unlink($link);
}
}
?>
tcknetwork dot com's solution for Win32 seemed attractive but darned if I could make it work so there seem to be some further notes needed. One obvious problem is in the line:
exec("linkd ".$p.$t." ".$p.$l);
Note that since $p (from the line above) is the absolute path to the directory of the current script, the $t and $l are only defined relative to that path. And should not contain any spaces. I tried putting single quotes around the link and target but it didn't help.
Olszewski_marek makes a good suggestion, but the readfile() function can also be used to obscure file downloads from end users.
/* Setup the file that will be sent */
$downloadDir = "some/secret/directory/";
$file = "theFileName.dat";
/* Required for IE, otherwise Content-disposition is ignored */
if(ini_get('zlib.output_compression')) ini_set('zlib.output_compression', 'Off');
/* Output HTTP headers that force "Save As" dialog */
header("Pragma: public");
header("Expires: 0");
header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
header("Cache-Control: private",false);
header("Content-Type: application/octet-stream");
header("Content-Disposition: attachment; filename=\\"$file\\";");
header("Content-Transfer-Encoding: binary");
header("Content-Length: ".@filesize($downloadDir.$file));
/* Prevent the script from timing out for large files */
set_time_limit(0);
/* Send the entire file using @ to ignore all errors */
@readfile($downloadDir.$file);
/* Exit immediately so no garbage follows the file contents */
exit;
Olszewski seems pretty good, but just to boost the security a bit, the fifth line of his script should read
<?
...
$q = rand(1,26);
...
?>
instead of rand(1,24).
To make your code portable on unix AND win32, do the following
1. Download http://www.dynawell.com/reskit/microsoft/win2000/linkd.zip
2. Unzip, put linkd.exe in C:\Windows\System32 or C:\WINNT\System32
3. Include in your code the following
<?
function _syslink($t /*target*/ ,$l /*link*/ ) {
if ($_SERVER["WINDIR"]) {
$p=dirname($_SERVER["SCRIPT_FILENAME"])."/";
exec("linkd ".$p.$t." ".$p.$l);
} else syslink($t,$l);
}
function _unlink($l /*link*/ ) {
if ($_SERVER["WINDIR"]) {
$p=dirname($_SERVER["SCRIPT_FILENAME"])."/";
exec("linkd ".$p.$l." /D");
} else unlink($l);
}
?>
4. Enjoy
_symlink(TARGET,LINK) works like symlink() on *nix
_unlink(LINK) to delete properly the link created
Olszewski's method of creating downloadable links on the fly is pretty good.
I use a different technique under Apache where if you want a file, you might use a url like:
mydomain.com/files/mysecretfile.doc
But in fact Apache redirects this to a script with a url like:
mydomain.com/utilities/downloadfile.php?filename=mysecretfile.doc
(The browser address bar will still show the first url).
The script downloadfile.php can then handle all the mucky stuff like checking session variables to see if someone is logged on, whether they have access to mysecretfile.doc, and if you want to encrypt before download.
An advantage of this is that the php code to achieve the equivalent of Olszewski's is shorter, so might execute faster. But more importantly, a casual hacker might think all he has to do is find myverysecurefile.doc under the /home/user/public_html/files/mysecretfile.doc, or copy the url - but there's nothing there! He won't find it or get an Apache error page.
This might sound obvious, but sometimes these details slip from our minds...
Make sure the directory in which you are trying to create the symbolic link (AND the directory you are trying to link to in the case of directory linking) has the proper permissions, or you will get a "permission denied" from PHP. If you use for example,
echo shell_exec("ln -sv $target $name")
you might get the correct "link created" message, but without the link being actually created.
It's probably the same thing with hard links, I just haven't tried it.
Faking Symlinks on Windows (Win2K + NTFS)
Every once in a while I'll bang my head against something really difficult. I needed to make symlinks, Windows won't let me. The workaround is a couple files in the Win2K Resource Kit called linkd.exe and delrp.exe
http://support.microsoft.com/default.aspx?scid=kb;EN-US;q205524
It only works on folders as far as I can tell, there is another program from SysInternals called Junction which will do both (although I only had success with files, not folders).
http://www.sysinternals.com/ntw2k/source/misc.shtml#junction
I hope this helps those who have experienced this same problem. One step closer to portable applications :)
Here is a simple way to control who downloads your files...
You will have to set: $filename, $downloaddir, $safedir and $downloadURL.
Basically $filename is the name of a file, $downloaddir is any dir on your server, $safedir is a dir that is not accessible by a browser that contains a file named $filename and $downloadURL is the URL equivalent of your $downloaddir.
The way this works is when a user wants to download a file, a randomly named dir is created in the $downloaddir, and a symbolic link is created to the file being requested. The browser is then redirected to the new link and the download begins.
The code also deletes any past symbolic links created by any past users before creating one for itself. This in effect leaves only one symbolic link at a time and prevents past users from downloading the file again without going through this script. There appears to be no problem if a symbolic link is deleted while another person is downloading from that link.
This is not too great if not many people download the file since the symbolic link will not be deleted until another person downloads the same file.
Anyway enjoy:
<?php
$letters = 'abcdefghijklmnopqrstuvwxyz';
srand((double) microtime() * 1000000);
$string = '';
for ($i = 1; $i <= rand(4,12); $i++) {
$q = rand(1,24);
$string = $string . $letters[$q];
}
$handle = opendir($downloaddir);
while ($dir = readdir($handle)) {
if (is_dir($downloaddir . $dir)){
if ($dir != "." && $dir != ".."){
@unlink($downloaddir . $dir . "/" . $filename);
@rmdir($downloaddir . $dir);
}
}
}
closedir($handle);
mkdir($downloaddir . $string, 0777);
symlink($safedir . $filename, $downloaddir . $string . "/" . $filename);
Header("Location: " . $downloadURL . $string . "/" . $filename);
?>
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