export mysql database to file

public function back() {
      $dbhost = 'localhost:3036';
      $dbuser = 'root';
      $dbpass = '';
      $dbname = 'tawrny_dr';
      $backup_file = $dbname . date("Y-m-d-H-i-s") . '.sql';
      //DO NOT EDIT BELOW THIS LINE
   //Export the database and output the status to the page
      $command = 'mysqldump --opt -h' . $dbhost . ' -u' . $dbuser . ' -p' . $dbpass . ' ' . $dbname . ' > ~/' . $backup_file;
      exec($command, $output = array(), $worked);
      switch ($worked) {
          case 0:
              echo 'Database <b>' . $dbname . '</b> successfully exported to <b>~/' . $backup_file . '</b>';
              break;
          case 1:
              echo 'There was a warning during the export of <b>' . $dbname . '</b> to <b>~/' . $backup_file . '</b>';
              break;
          case 2:
              echo 'There was an error during export. Please check your values:<br/><br/><table><tr><td>MySQL Database Name:</td><td><b>' . $dbname . '</b></td></tr><tr><td>MySQL User Name:</td><td><b>' . $dbname . '</b></td></tr><tr><td>MySQL Password:</td><td><b>NOTSHOWN</b></td></tr><tr><td>MySQL Host Name:</td><td><b>' . $dbhost . '</b></td></tr></table>';
              break;
      }
      die();
  }

ERROR: Exec is not available!!!"