Programmatically append to file in PHP - php

Question

I am trying to programmatically append an RSA public key to the authorized_keys file through a website and haven't been able to make any solutions I found work. I have tried using PHP's file_put_contents() function but I run into a permission denied error, and I have a python script that works, but I cannot seem to get PHP to execute it with either the exec() command or shell_exec(). Here's the relevant PHP code:
if(#$_POST['action']=='submit'){
$key = $_POST['key_field'];
//file_put_contents("/home/biosproject/.ssh/authorized_keys", $key, FILE_APPEND);
$test = "/usr/bin/python savetofile.py \"".$key."\"";
$tmp = shell_exec($test);
}
I'm aware that I need to sanitize the input but the site is currently in development so I'm just testing it like this in the meantime. Right now I'm using XAMPP which runs Apache. Is there something I'm missing or could try? For the PHP exec/shell_exec, I have tried using the full pathnames for all parts of the command, but nothing has worked yet. The python script is as follows:
#!usr/bin/python
import sys
key = sys.argv[1]
with open("/home/biosproject/.ssh/authorized_keys","a") as append:
diditwork = append.write(key)
print key
As I mentioned before, this script is functional, but I can't call it from the PHP script.
EDIT:
My authorized_keys file looks like so: -rw-rw-rw- 1 biosproject www-data 1200 Apr 15 13:17 /home/biosproject/.ssh/authorized_keys
UPDATE:
I fixed the problem by bypassing permissions using a cron job that appends the necessary information from a database entry instead. Works great now!

Answer 1

The Python script won't help you here - it's a permissions issue with the /home/biosproject/.ssh/authorized_keys file, i.e. Apache doesn't have permission to modify it, and nor will any process it spawns, which would include your Python script.
Simplest fix would be to change the file permissions so it's writable by Apache. Assuming apache runs as group www-data, do...
sudo chgrp www-data /home/biosproject/.ssh/authorized_keys
sudo chmod g+w /home/biosproject/.ssh/authorized_keys
...although I forget if ssh complains if authorized_keys is set to g+w.
Update
It occurs to me that www-data will also need +x access to all parent directories of /home/biosproject/.ssh/authorized_keys to be able to change it, although I'm pretty sure that ssh will complain if you change the .ssh directory permissions in this way.
You'll either have to run apache with the same UID as the owner of the /home/biosproject/.ssh directory, or use a setuid script to make the changes.

Answer 2

Explanation about my inline code:
$text = "nice text to append :P";
// open a file handler with a+ flag that means "open file for append and if it does not exist, create it"
$fo = fopen("filename.ext", "a+");
// append $text to file handler with a \n at the end
fwrite($fo, $text . PHP_EOL);