which command do not work

I'm trying to deploy app on AWS under `nginx` user. Line 

``` php
        $path = trim(shell_exec('which '.$command));
```

return empty result. After some debug I catch the problem.  The `PATH` variable is not exported. So the line:

``` php
        $path = trim(shell_exec('export PATH=$PATH; which '.$command));
```

is working for. How about fix it?
