Something

$categories = array();

foreach($_POST AS $key => $value) {
   if (is_int(strpos($key,"subcat"))) {
      $t = explode("_",$key);
      $a = $t[1];
      $b = $t[2];
      $c = $t[3];
      
      $categories[$a][$b][$c] = $value;
   }
}