<?php

function convertFigure($figure) {
    $start = 0;
    $parts = array();
    $increase_start = array(0, 5, 10, 15, 20);
   
    for($x = 0; $x < 10; $x++) {
            $length = (in_array($start, $increase_start)) ? 3 : 2;
            $parts[$x] = substr($figure, $start, $length);
            $start = $start + $length;
    }
   
    return 'hr-'.$parts[0].'-'.$parts[1].'.hd-'.$parts[2].'-'.$parts[3].'.ch-'.$parts[4].'-'.$parts[5].'.lg-'.$parts[6].'-'.$parts[7].'.sh-'.$parts[8].'-'.$parts[9];
}


echo convertFigure("8281718002275052950523009"); // app.txt
 
?>