posix_setuid()

(PHP 4, PHP 5, PHP 7)

Set the UID of the current process

说明

Set the real user ID of the current process. This is a privileged function that needs appropriate privileges (usually root) on the system to be able to perform this function.

参数

The user id.

返回值

成功时返回 TRUE， 或者在失败时返回 FALSE。

范例

posix_setuid() example

This example will show the current user id and then set it to a different value.

参见

• posix_setgid()Set the GID of the current process
• posix_seteuid()Set the effective UID of the current process
• posix_getuid()Return the real user ID of the current process
• posix_geteuid()Return the effective user ID of the current process

Note that on unix, if your target user does not have a valid shell, some php functions (eg: tempnam) will not work correctly:
$ grep www-data /etc/passwd
www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin
$ cat test.php 
#!/usr/bin/php -q

$ sudo ./test.php 
PRE SetUID: /tmp/somethingrsb1qZ
POST SetUID:

In response to a note above that advocated the user of system() in a setuid program written in C, this is generally a bad idea for security. 
You should use the standard library calls like execl() instead because system() can be manipulated to execute the wrong thing using the SHELL, IFS and possibly other variables.

When you do a posix_setuid from root to some other users you will not have access to files owned by root according to their permissions. For instance if you change owner of the process and still need to open a file for read or write with 600 permission owned by root you will receive a permission denied.
There are some ways to do this (i.e. a unix socket or tcp daemon etc), but probably the most easiest way is:
Open the file before changing ownership of process, save the file pointer in a global variable and use it after changing ownership. 
For example assume /root/test_file is a file owned by root:root and have a permission of 600 and you are running this script under root. This code will not work:

But this one will work:

Hope this helps some one.
[Tested on CentOS 5 - Linux 2.6.x - PHP 5.2.x]

On many UNIX systems (tested on Debian GNU/Linux), SUID is disabled for scripts and works only for binaries. If you need to setuid, you must use a wrapper binary that runs setuid() php script. Here's an example:
$ nano suexec.cpp
#include 
using namespace std;
int main()
{
system("php /home/php/php_user.php");
return 0;
}
$ g++ -o suexec suexec.cpp
$ sudo chown root:root suexec
$ sudo chmod 4755 root
Then we create short PHP script to set process uid (you should already know how to do this). Don't even try to experiment with auto_prepend_file in php.ini, it doesn't work as expected.

It seems like this function returns true if you try to change uid to the already active user - even if you aren't root.
Should save you one if-statement in some cases.

For simple operations, you can easily create a privilege-separation mechanism to perform commands that require elevated privileges.
For example, in creating a document repository, I had the need to provide access to certain directory and file operations based on a user's login name. It's unrealistic and unsecure to provide the web server access to all of the directories that the user may need to access, so I created a setuid() script to perform the required operations for me.
An exerpt from the code demonstrates this:

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