Grunt没有运行Php shell_exec（）

The Goal: To run a grunt process in a php project on a button click. I have one gruntfile to handle the build process for multiple Wordpress themes. It works, and I pass in a theme name from a drop down on a form, as grunt.option('themeKey').

Issue: I have a working command that works in Terminal and a button to fire it, but it doesn't actually run on the button click.

    <?php
      if(isset($_POST['formTheme'])){
        $grunttheme = $_POST['formTheme'];
        $process = 'cd /Applications/XAMPP/xamppfiles/htdocs/grunt && grunt build --themeKey="'.$grunttheme.'"';
        echo shell_exec($process);
      }
    ?>

Error: sh: grunt: command not found

What do I need to do to make this command run from index.php? I've tried a few different options pulled from different php documentation and stackoverflow entries, like aliasing, PATH, etc. I'm new with both php and still pretty green with command line, so I might just have bad syntax or be missing an important chunk. Any help is appreciated, thanks!