PHP exec() not working - permission error?

I'm trying to use exec() to run a program in /var/www/litecoin/bin and echo the output.

Basically, this: (from /var/www/html/index.php)

<?php
   echo exec("../litecoin/bin/litecoin-cli getinfo");
?>

It just shows a blank page, though.

I know it's probably a permission error, as running ls works fine, but I have no idea how to fix it. (or if there's a better way of doing thing)

Running the command directly via the terminal works fine. I'm using PHP7.0 and Apache on Ubuntu 16.04, if it matters.

Edit

It works fine when I run it via the terminal doing php index.php when logged into www-data, but when I open it in the web browser it doesn't seem to execute /var/www/litecoin/bin/litecoin-cli at all.

Edit 2

When redirecting errors from stderr to stdout (2>&1), I get the following:

[0] => 
[1] => 
[2] => ************************
[3] => EXCEPTION: N5boost10filesystem16filesystem_errorE
[4] => boost::filesystem::create_directory: Permission denied: "/.litecoin"
[5] => litecoin in AppInitRPC()
[6] =>

www-data has full permission to /var/www/ though.

Solution

The major difference in such cases i find is the environment and the permissions. The error [4] => boost::filesystem::create_directory: Permission denied: "/.litecoin" suggests that your php page is looking for the folder litecoin in ~ or $HOME and it is not set. That's why it becomes /.litecoin

I created a simple index.php file with below code

<?php

$out = array();
exec("env", $out);

var_dump($out);

The output of same on my default php is

/home/vagrant/nginx/html/index.php:6:
array (size=9)
  0 => string 'APACHE_RUN_DIR=/var/run/apache2' (length=31)
  1 => string 'APACHE_PID_FILE=/var/run/apache2/apache2.pid' (length=44)
  2 => string 'PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin' (length=65)
  3 => string 'APACHE_LOCK_DIR=/var/lock/apache2' (length=33)
  4 => string 'LANG=C' (length=6)
  5 => string 'APACHE_RUN_USER=www-data' (length=24)
  6 => string 'APACHE_RUN_GROUP=www-data' (length=25)
  7 => string 'APACHE_LOG_DIR=/var/log/apache2' (length=31)
  8 => string 'PWD=/home/vagrant/nginx/html' (length=28)

As you can see there is $HOME and that could be one possible reason causing a issue

So you should make sure the correct environment is available to the executable that you are running. You can do that by doing

exec("HOME=/var/www/html X=Y A=B env", $out);

The output of the command shows it can see the updated variables

/home/vagrant/nginx/html/index.php:6:
array (size=12)
  0 => string 'HOME=/var/www/html' (length=18)
  1 => string 'APACHE_RUN_DIR=/var/run/apache2' (length=31)
  2 => string 'APACHE_PID_FILE=/var/run/apache2/apache2.pid' (length=44)
  3 => string 'A=B' (length=3)
  4 => string 'X=Y' (length=3)
  5 => string 'PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin' (length=65)
  6 => string 'APACHE_LOCK_DIR=/var/lock/apache2' (length=33)
  7 => string 'LANG=C' (length=6)
  8 => string 'APACHE_RUN_USER=www-data' (length=24)
  9 => string 'APACHE_RUN_GROUP=www-data' (length=25)
  10 => string 'APACHE_LOG_DIR=/var/log/apache2' (length=31)
  11 => string 'PWD=/home/vagrant/nginx/html' (length=28)

Figure out which environment variables you need and make sure they exists and try to set the paths so your executable used /var/www or /var/www/html whichever path it has access to and then executes the program