The following line was the best I read to this problem: "You need to disable safe mode in the php.ini located in the \Apache2\bin folder, not the php folder. Restart Apache2 service right after to load the new settings."

The solution is:

1. stop all services from Wamp-Server and close the programm

2. Open .../wamp/bin/apache/Apache2../bin/php.ini

3. copy php.ini to desktop and open it

4. set safe_mode_exec_dir (line after = is empty, so IT IS ON!!!) set it off!

5. save

6. copy back to dir (maybe you need admin rights)

7. start wamp-server

8. enjoy exec() and co.

Thank you very much for your input! As it turns out, it was Windows issue caused by the 'Interactive Services Detection' feature. Apache was running as a system service, which prevented calls to external programs (with a GUI). I disabled the run-as-service feature in XAMPP, which solved the problem. A more thorough explanation can be found here: http://php.net/manual/en/book.exec.php

1) Check the security setup

Can shell_exec() be run or not? If it can't then it's because you are having some security settings enabled.

The problem is that shell_exec() and exec() can be disabled for security reasons. This can be changed by editing PHP's disable_functions option.

INI settings are usually stored in specific *.ini files under the /etc/php/* directories. So you have to look for some settings in there. You’ll find several INI settings, depending on how PHP is run. To find them :

find /etc/php/ -iname 'php.ini'

This could output something like this :

/etc/php/8.2/fpm/php.ini
/etc/php/8.2/apache2/php.ini
/etc/php/8.2/cli/php.ini
/etc/php/8.2/cgi/php.ini

To see the settings and which INI file is loaded, add this at the beginning of your script and call it via your web server :

phpinfo();
die();

You'll see all the loaded INI files and what is set on the disable_functions option.

/etc/php/8.2/cli/php.ini is probably setting disable_functions to an empty string, meaning that in command line there's no restriction. But for the other INI files, this option might contain a list of functions that you won't be able to run. Edit this option if needed. But be always aware that giving your server the ability to run some binaries can lead to some security issues if your app is taking some user input to build the command.

Apache configuration can also set some PHP settings, just for a specific Virtual Host. So have a look at them if you can't find something in the php.ini files.

Depending on your PHP setup, you may also have to check safe_mode and suhosin.executor.* if you have the Suhosin hardening module installed.

2) Verify that you can run shell_exec

There are several ways to run a binary. shell_exec is equivalent to the built-in backtick operator. So if shell_exec is listed in the disable_functions option then you won't be able to use the backtick operator.

If you get an error 500 then it's typically because you cannot run shell_exec. If you check /var/log/nginx/error.log or /var/log/apache2/error.log with this command :

sudo tail -f /var/log/{apache2,nginx}/error.log

It will print both logs until you press CTRL+C. You may see this error :

PHP Fatal error:  Uncaught Error: Call to undefined function shell_exec()

PHP also offers exec(), passthru(), system(), and the proc_*() functions. So you've got several possibilities to try and run your command.

Have a try with a simple PHP code :

<?php

header('Content-Type: text/plain; charset=utf-8');

echo 'PHP user given with the "id" command: ' . `id`; // equivalent to shell_exec('id');
echo 'System date is ' . `date --iso-8601=seconds`;

This should output RAW text in your browser with :

PHP user given with the "id" command: uid=33(www-data) gid=33(www-data) groups=33(www-data)
System date is 2023-09-29T11:50:28+00:00

3) Use a full path to the binary

When shell_exec() is run from the web server, it's rather common that the binary isn't found in the PATH. Usually, the www-data user running PHP won't even have a shell. But this user can normally run some binaries, except if some settings at point 1 is blocking it.

To find the full path to a binary, use the which command :

which wkhtmltopdf

This should output the full path to the binary. Certainly something such as /usr/bin/wkhtmltopdf.

Now replace your PHP code with the full path to the binary:

header('Content-Type: text/plain; charset=utf-8');
echo shell_exec('/usr/bin/wkhtmltopdf --version');

Does it print wkhtmltopdf 0.12.6 like expected?

If no, then we have to investigate more at the next point.

4) Get the execution output and find errors

This can be done with the help of exec(), passthru() or system() as they all return the execution status code.

With the proposition of Volkerschulz, we'll also redirect the standard error stream to the standard output stream so that we can see some details that may have only be printed to the error stream.

This is done by redirecting stderr (2) into stdout (1) by adding 2>&1 after your command.
Just as info, stdin is 0.

In PHP, this becomes :

<?php

header('Content-Type: text/plain; charset=utf-8');

$command = '/usr/bin/wkhtmltopdf --wrong-option 2>&1';

$last_line = exec(
  $command,    // The command to execute.
  $output,     // A variable that will be filled with an array of all the lines returned.
  $result_code // The return status of the executed command.
);

var_export([
  '$command' => $command,
  '$output' => $output,
  '$last_line' => $last_line,
  '$result_code' => $result_code,
]);

This outputs :

array (
  '$command' => '/usr/bin/wkhtmltopdf --wrong-option 2>&1',
  '$output' => 
  array (
    0 => 'Unknown long argument --wrong-option',
    1 => '',
    2 => 'Name:',
    3 => '  wkhtmltopdf 0.12.6',
    ...
    ...
    95 => '',
  ),
  '$last_line' => '',
  '$result_code' => 1,
)

Now I'm able to see that I got a $result_code of 1 instead of 0.

In case the executable is having other errors, you'll see them in the $output array as the stderr stream should be visible. This should help you find how to fix the issue. Probably some other security problems due to the fact the www-data user might be missing some rights, paths or whatever.

PS: On my Vagrant box of Ubuntu 22.04, I managed to run wkhtmltopdf without errors on both Apache and NGINX.

Don't forget also that your www-data user should have write access to the destination folder where you are producing the PDF file.

5) Try re-installing wkhtmltopdf and/or use a PHP wrapper

You might have to re-install wkhtmltopdf if it's not properly working :

sudo apt remove wkhtmltopdf
sudo apt autoremove
sudo apt install wkhtmltopdf

You can also try using a PHP wrapper library for wkhtmltopdf called Snappy. It might be useful.

6) Switch to an alternative in pure PHP if necessary

Perhaps you won't manage to make wkhtmltopdf work that easily. It's a pity as it's effectively a fast and reliable solution.

But to create a PDF from HTML you could also use the Dompdf PHP library. I've used it in the past and was happy with it.

The PHP shell_exec will try to run the command in the host system. Therefore you cannot run linux command on windows.

If you really want to try running that in windows and you have windows 10 maybe you can try using wsl ( windows subsystem linux )