ini_set('display_errors',1);
error_reporting(E_ALL);
echo "<pre>";
  echo getcwd()."\n";
  echo exec('whoami')."\n";
  $file = fopen('counter__.txt', 'w');
  print_r($file);
  if($file ) print_r(fstat($file));
echo "</pre>";
?>

/var/www/html/bitrix
nginx
Warning:  fopen(counter__.txt): failed to open stream: Permission denied in /var/www/html/bitrix/etst.php on line 7

sudo -su nginx
cd ~
cd /var/www/html/bitrix
touch ttt