shell_exec(); not working properly in PHP
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I'm trying to execute a command thru shell_exec(); but it does nothing. I know shell_exec(); is enabled because these work perfectly:

    echo shell_exec('which php');
    echo shell_exec('ls');

They do output correctly.

The problem is when I want to execute something like this:

    shell_exec("php file.php var1 var2 > /dev/null &");

It does nothing, but when I run it thru the Terminal on my computer, it does work.
I have also tried doing it with the full path like this:

    shell_exec("/usr/bin/php /full/path/file.php var1 var2 > /dev/null &");

And it doesn't work, but again... it works if I run it thru my Terminal's computer.

The php.ini safe mode is off, and there are no disabled functions on it, so I don't know what could be happening. And yes, I have access to SSH and got a dedicated IP.