fwrite and sort

i have a form with checkboxes which after POST it writes to a file. i would like to allow the user to sort the selected checkbox filename in the order they want it written to file. so i was thinking creating an input text box where they can enter a number, then have it written to file in that order.. question is how? the script starts like such: foreach($_POST['map'] AS $file) { if(file_exists('/maps/'.trim($file))) { $found .= $file; } } if (file_exists($mapcycle) && $found) { $fp = fopen($mapcycle, 'w'); fwrite($fp, $found); fclose($fp); } any help is appreciated.